Clean up layout of main page and add shortcut table

The idea here is to clean up the layout of the main page and provide a more useful UI for users. I now show a table that gives examples of the "shortcuts" that can be used in the main "Go" form. I have also added shortcuts for a GH username or username/repo.

![screen shot 2014-04-09 at 9 24 09 pm](https://cloud.githubusercontent.com/assets/27600/2663901/3122504e-c068-11e3-9ae1-eb2b9b8df6f0.png)