Append tax information to pricing object

Append a bool `hasTax` to the Pricing object to help with templates and such.

Append the tax information to a `taxes` array on the Pricing object.

Add some tests.

(Also, you should note that you have tests at https://github.com/recurly/recurly-js/blob/master/test/unit/recurly/coupon.test.js that are not listed https://github.com/recurly/recurly-js/blob/65983710cde1354dc6502864487b32ae920ea0bf/test/index.js so it is never run... this may be on purpose...)