Preferences, error handling

Fixes #23 

Looks like this if you screw everything up:
![screen shot 2014-07-02 at 12 42 30 am](https://cloud.githubusercontent.com/assets/421341/3450043/f60b21f2-0171-11e4-9606-f01eb8a84f2b.png)

There's no more "bad url" sheet, the window just shakes if you try to save with invalid input. When bad inputs are corrected, the orange icons instantly turn into a quasi-invisible checkmark.

I'm gonna merge this, but let me know if you think it could've done better, I'll try to improve it.