Remove login via Google OmniAuth

Closes #661.

## Before
### Login
![screen shot 2014-10-02 at 11 42 42 am](https://cloud.githubusercontent.com/assets/667753/4492868/c9700b46-4a44-11e4-8112-66cdae6e4e79.png)
### Editing profile
![screen shot 2014-10-02 at 12 05 13 pm](https://cloud.githubusercontent.com/assets/667753/4492957/8dfe0440-4a45-11e4-9c3f-d3ac368f4769.png)

## After
### Login
![screen shot 2014-10-02 at 11 46 05 am](https://cloud.githubusercontent.com/assets/667753/4492873/cfd704c6-4a44-11e4-9234-2c4a7d4efdb2.png)
### Editing profile
![screen shot 2014-10-02 at 12 03 06 pm](https://cloud.githubusercontent.com/assets/667753/4492919/3fffe894-4a45-11e4-8375-fea056253cdf.png)