Adds preview plugin for gallery

Fixes #286 

Modified to use preview plugin with caching https://github.com/sul-dlss/SearchWorks/pull/309
Adds preview plugin for gallery building off of #249 

![screen shot 2014-06-16 at 5 17 30 pm](https://cloud.githubusercontent.com/assets/1656824/3294994/d709d640-f5b4-11e3-9a2b-b78a2c6ae008.png)