Added new theme - Clean

Great extension, but I feel that the jQueryUI styles detract from the experience.

Added new theme: Clean

Many enhancements to spacing, contrast, and legibility. Replaced all glass effects with flat colours. 

Updated font with a light font from Google webfonts.


Only change outside of theme files is to default tweet colours in options_backend.js.

Reasoning behind this is that the extension assumes that the defaults for tweet colours will not conflict with the themes. I feel that the user should have the option of adding colours to their tweets should they feel it improves their experience, but should not be provided defaults which are likely to not match a theme.

![Screen Shot 2013-01-12 at 1 53 04 PM](https://f.cloud.github.com/assets/1510520/61955/ac48e734-5cae-11e2-98e2-b66c68732ea4.png)