Cleanup ui

## Merge this first
(Depends on @jules2689's PR: https://github.com/joshpc/Testosaurus/pull/26)

## What this does
Cleans up the UI in a few parts. Changes the colour palette of the application a little. You can see the new colours in `NSColor+TestasaurusColors.h`.

![NewLook](http://dl.dropbox.com/u/1512544/Screenshots/t.png)

**Note** This adds the first retina graphics for the plus and delete buttons. This also cleans up the buttons on the table header view.

@jules2689 @j-mutter 

## Ship list
- [ ] Code Review
- [ ] Test