Selections dropdown and recently selected

Fixes #39 

Adds two new js plugins.  One updates the count in the selections dropdown, the other grabs recent selections from the new `RecentSelectionsController` and updates the dropdown list.

![screen shot 2014-06-14 at 10 38 35 am](https://cloud.githubusercontent.com/assets/1656824/3279109/c1afae62-f3ea-11e3-9279-ef366696fee1.png)

![screen shot 2014-06-14 at 10 38 41 am](https://cloud.githubusercontent.com/assets/1656824/3279115/cd307e56-f3ea-11e3-9a5d-3d1b134d54d6.png)

`#map` can take a method as a block parameter instead of passing a block directly so `#map { |x| x.document_id }` and `#map(&:document_id)` are the same.