reorganize homepage to be consistent with design

Closes #69 
Closes https://github.com/sul-dlss/earthworks/issues/47
Closes https://github.com/sul-dlss/earthworks/issues/4

Categories will be better when the metadata gets better. Automatically chooses top subjects (this could be extended later).
![screen shot 2014-11-17 at 4 07 43 pm](https://cloud.githubusercontent.com/assets/1656824/5080372/e41d8c2c-6e73-11e4-8dce-022e48db28bd.png)