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)

I believe that the only place that the clear bookmarks button exists in Blacklight is on the Bookmarks page.  Given this, Blacklight (perhaps erroneously) redirects explicitly to `Bookmarks#index` ( https://github.com/projectblacklight/blacklight/blob/master/app/controllers/bookmarks_controller.rb#L109 ).

@jvine is that behavior okay or do we need to override this behavior to redirect back to the place they cleared the list from?