Adds Pricing support for `tax_code`

- Adds Pricing.prototype.tax
- Adds tax_code to Pricing.prototype.attach
- Documents tax_code for Recurly.prototype.tax

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Updates duo build command for compatiblity

- Updates component/type along the way

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Merge pull request #166 from recurly/tax

Adds Pricing support for `tax_code`