Added emails tab to settings + email tags class + first pass at registra...

...tion email

related: https://github.com/affiliatewp/AffiliateWP/issues/266

First pass at new emails settings tab
![screen shot 2014-06-23 at 4 05 01 pm](https://cloud.githubusercontent.com/assets/52581/3353761/9771ee42-fa8b-11e3-84eb-e951036aecf9.png)