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...)  
This is a little tricky, and I might prefer to instead just set this before iterating on taxes. Thus...

```js
self.price.taxes = [];
each(taxes, function (tax) {
  // ...
  self.price.taxes.push(tax)
});
```

That way the pricing output object consistently has a taxes array, and a merchant won't need to check for its presence before working with it.

As for hasTax, I am somewhat hesitant to include this, as it can be easily surmised by counting the taxes array, given it is always present.