Add support for invoice branding themes

Xero API provides support for custom [branding themes](http://developer.xero.com/documentation/api/branding-themes/). So I decided to add support for this feature to the gem as well.
![screen shot 2014-04-29 at 11 41 30](https://cloud.githubusercontent.com/assets/2370/2827458/5a4e6e42-cf83-11e3-8eff-af8fce3afd70.png)

*P.S. Also all specs are green now  :smiley:*