Favorite trades select

Add the favorite trades dropdown menu.
Add pair action in menu.
Form defaults to the last favorite trade placed.

Fixes #761.

![output_nqfqme](https://cloud.githubusercontent.com/assets/3108007/4398511/87b6550e-4457-11e4-9767-a3fc4ba16fd6.gif)



This seems like it should be part of the Gateways service