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)

Also, I believe that both Show and Clear are intended to be disabled when there are no selections at all.

![no-selections](https://cloud.githubusercontent.com/assets/96776/3304003/2252c904-f646-11e3-9d0d-a74649c8f8f8.png)