Add simple selector for switching between 2 and 4 space tabs

Nothing too fancy, but what do you think of this approach? I'd like to get some feedback, then later add in autodetection and per-tab spacing.

![screen shot 2014-02-28 at 6 24 09 pm](https://f.cloud.github.com/assets/106826/2300287/b0561d18-a0e8-11e3-8ff7-cfca38067eb7.png)

Also - no tests yet and the selector is set to show on `ctrl-alt-p` but that's just a random key I chose. Feel free to change it.