Adds ability for admin to search payments by email

This introduces the ability to search campaign payments by email in addition to the existing "search by ID" functionality.

![screen shot 2013-12-11 at 1 41 25 pm](https://f.cloud.github.com/assets/1132438/1728413/45eed730-62ae-11e3-9207-6f136197c3a5.png)