Add repository management to Cakebrew

This pull request adds basic repository management (tap/untap) to cakebrew.
Repository listing:
![screen shot 2014-08-29 at 10 56 34](https://cloud.githubusercontent.com/assets/2173772/4088836/ddbe2ec2-2f62-11e4-8766-35f756eaa3a0.png)
Tapping a new repository:
![screen shot 2014-08-28 at 10 32 28](https://cloud.githubusercontent.com/assets/2173772/4073580/654b608a-2e96-11e4-8653-2e32244860bf.png)
![screen shot 2014-08-28 at 10 32 42](https://cloud.githubusercontent.com/assets/2173772/4073582/67e1012e-2e96-11e4-913f-6f193ce70d3e.png)
Untapping a repository:
![screen shot 2014-08-28 at 10 32 15](https://cloud.githubusercontent.com/assets/2173772/4073585/6dcf446a-2e96-11e4-8508-9e510cd35ef0.png)