Improve the design of the tab completion UI

This PR improves on the style of the tab completion UI by doing the following:

* Rounded corners
* White background
* Font matches that of the CM editor
* Horizontal alignment with the matched text.

Here is a sample.

![Screen Shot 2013-04-29 at 2 02 48 PM](https://f.cloud.github.com/assets/27600/441157/d56cc458-b110-11e2-8c20-9afc961ff2c1.png)