Add `.npmignore` to create a leaner package
Add `.npmignore` to create a leaner package

Adds an `.npmignore` file to the project so that the final package sent
to NPM is lighter. Tests, examples, and other resources are stripped out
(it's easy to link back to this repository to refer to these if needed).

I've written this is in an "inverted" style so that only by default
everything is blacklisted, and we add back in the files and folders that
we know we'll want.

I verified that this does the right thing by using `npm pack`. Before
the addition of `.npmignore`:

``` sh
$ npm pack && tar -zxf stripe*.tgz && ls -R package/
stripe-6.1.0.tgz
CHANGELOG.md    README.md       examples/       package.json
LICENSE         VERSION         lib/            test/

package//examples:
webhook-signing/

package//examples/webhook-signing:
express.js      package.json

package//lib:
Error.js                        StripeMethod.js                 resources/
MultipartDataGenerator.js       StripeResource.js               stripe.js
StripeMethod.basic.js           Webhooks.js                     utils.js

package//lib/resources:
Accounts.js                     EphemeralKeys.js                Recipients.js
ApplePayDomains.js              Events.js                       Refunds.js
ApplicationFeeRefunds.js        ExchangeRates.js                SKUs.js
ApplicationFees.js              FileUploads.js                  Sources.js
Balance.js                      InvoiceItems.js                 SubscriptionItems.js
BitcoinReceivers.js             Invoices.js                     Subscriptions.js
ChargeRefunds.js                IssuerFraudRecords.js           ThreeDSecure.js
Charges.js                      LoginLinks.js                   Tokens.js
CountrySpecs.js                 OrderReturns.js                 Topups.js
Coupons.js                      Orders.js                       TransferReversals.js
CustomerCards.js                Payouts.js                      Transfers.js
CustomerSubscriptions.js        Plans.js                        UsageRecords.js
Customers.js                    Products.js
Disputes.js                     RecipientCards.js

package//test:
Error.spec.js           flows.spec.js           stripe.spec.js
StripeResource.spec.js  mocha.opts              testUtils.js
Webhook.spec.js         resources/              utils.spec.js

package//test/resources:
Account.spec.js                 EphemeralKeys.spec.js           Recipients.spec.js
ApplePayDomains.spec.js         Events.spec.js                  Refunds.spec.js
ApplicationFeeRefunds.spec.js   ExchangeRates.spec.js           SKUs.spec.js
ApplicationFees.spec.js         FileUploads.spec.js             Sources.spec.js
Balance.spec.js                 InvoiceItems.spec.js            SubscriptionItems.spec.js
BitcoinReceivers.spec.js        Invoices.spec.js                Subscriptions.spec.js
ChargeRefunds.spec.js           IssuerFraudRecords.spec.js      ThreeDSecure.spec.js
Charges.spec.js                 LoginLinks.spec.js              Tokens.spec.js
CountrySpecs.spec.js            OrderReturns.spec.js            Topups.spec.js
Coupons.spec.js                 Orders.spec.js                  TransferReversals.spec.js
CustomerCards.spec.js           Payouts.spec.js                 Transfers.spec.js
CustomerSubscriptions.spec.js   Plans.spec.js                   UsageRecords.spec.js
Customers.spec.js               Products.spec.js                data/
Disputes.spec.js                RecipientCards.spec.js

package//test/resources/data:
minimal.pdf
```

And after:

``` sh
$ npm pack && tar -zxf stripe*.tgz && ls -R package/
stripe-6.1.0.tgz
CHANGELOG.md    LICENSE         README.md       VERSION         lib/            package.json

package//lib:
Error.js                        StripeMethod.js                 resources/
MultipartDataGenerator.js       StripeResource.js               stripe.js
StripeMethod.basic.js           Webhooks.js                     utils.js

package//lib/resources:
Accounts.js                     EphemeralKeys.js                Recipients.js
ApplePayDomains.js              Events.js                       Refunds.js
ApplicationFeeRefunds.js        ExchangeRates.js                SKUs.js
ApplicationFees.js              FileUploads.js                  Sources.js
Balance.js                      InvoiceItems.js                 SubscriptionItems.js
BitcoinReceivers.js             Invoices.js                     Subscriptions.js
ChargeRefunds.js                IssuerFraudRecords.js           ThreeDSecure.js
Charges.js                      LoginLinks.js                   Tokens.js
CountrySpecs.js                 OrderReturns.js                 Topups.js
Coupons.js                      Orders.js                       TransferReversals.js
CustomerCards.js                Payouts.js                      Transfers.js
CustomerSubscriptions.js        Plans.js                        UsageRecords.js
Customers.js                    Products.js
Disputes.js                     RecipientCards.js
```

Fixes #468.

stripe/stripe-node,stripe/stripe-node