Add tests.
Add tests.

- Add invalid date test.
- Add simple sign/verify test.
- Use fixed past date headers.
- Ignore clock skew so a fixed past date can be used to check signature
  output.
- Check Authorization header results.

arekinath/node-http-signature,LeisureLink/node-http-signature,joyent/node-http-signature,Aloomaio/node-http-signature,bsletten/node-http-signature,DinoChiesa/node-http-signature