Fix blacklight helper overrides - re closes #167

It appears that you need to inlcude the Hydra blacklight helpers which in turn include the Blacklight blacklight helpers in order to retain full hydra functionality when overriding or extending blacklight helper funcitonality.

See https://github.com/projectblacklight/blacklight/wiki/Configuring-and-Customizing-Blacklight#custom-view-helpers
And https://github.com/projecthydra/hydra-head/blob/master/hydra-core/app/helpers/hydra/blacklight_helper_behavior.rb
Merge pull request #193 from curationexperts/fix_thumbnails

Fix blacklight helper overrides - re closes #167