Change backend lookup path to custom action.

When making json requests agains the index action the
search context for the user is updated.  This causes the
"Back to Search" link and the pagination on the record view
to use the context of the last ajax request made from the
additional results block on the result set that the record came from.
Merge pull request #244 from sul-dlss/backend-lookup-change

Change backend lookup path to custom action.