Add paymentsource.Client to api.Client
Add paymentsource.Client to api.Client

api.Client doesn't provide access to paymentsource.Client and
paymentsource.Client doesn't provide a constructor similar to
api.Client.

froatsnook/stripe-go,stripe/stripe-go,ChannelMeter/stripe-go,stripe/stripe-go,ChannelMeter/stripe-go,froatsnook/stripe-go