Mavericks style background

Changes the background from __bg.jpg__ to use __-webkit-radial-gradient__ with the colors from Mavericks.

Looks like this:
![screen shot 2014-05-04 at 11 02 43 am](https://cloud.githubusercontent.com/assets/760249/2873665/a9bba24c-d3b6-11e3-9ac7-3db336ae2a2b.png)

Mavericks login for reference:
![screenshot from arstechnica](http://cdn.arstechnica.net/wp-content/uploads/2013/10/login-window@2x.png)