Users can follow categories

![screen shot 2014-10-15 at 05 00 40 pm](https://cloud.githubusercontent.com/assets/154595/4652600/4631477e-54a6-11e4-856f-8d4542e1b47d.png)

![screen shot 2014-10-15 at 05 02 20 pm](https://cloud.githubusercontent.com/assets/154595/4652601/466c8a8c-54a6-11e4-8acc-dae0ba5066de.png)

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