Display thumbnail in search results.

Fixes ndlib/planning#339

Refactor representative object

Refactoring thumbnail not to fetch from fedora each time

Remove representative_object method as its not needed

Removing duplicate view

Refactor Generic File to be represented

Make the first file as representative file
Get the tests passing
Merge pull request #316 from projecthydra/339-display-thumbnail-in-search-results

Display thumbnail in search results.