Fix powerline separators and make them configurable

This fixes #18. Powerline dividers now render at the right height on emacs cocoa, they are still aliased but that can be another PR.

This also allows the powerline separator to be configured by the user using the `powerline-default-separator` variable that is built into powerline.

The problem was that to fix another bug powerline stopped calculating the height of the bar and used the font height for the separators. I think one of the fancy spacemacs things made the bar higher than the font height, causing the separators to be the wrong height. This was fixed by setting the `powerline-height` variable to a reasonable value.

The other changes are unrelated to the bug, they just make the theme use the `powerline-default-separator` variable so that it can be changed by the user. My editor (currently ST3, but that will change soon) also trimmed all trailing whitespace in the file, not a bad thing.

**Edit:** I also fixed the aliasing a bit. See my comment for an updated screenshot.

### Screenshot:
![screen shot 2014-10-18 at 10 54 49 am](https://cloud.githubusercontent.com/assets/887610/4690000/bc3ae2a4-56d6-11e4-9c67-3842c23bbbb7.png)
(taken after height fix but before aliasing fix.)