Make OAI's implementation become generic and overridable.
Make description optional.
Merge branch '7.x' of github.com:Islandora/islandora_oai into 7.x-USC-11

Conflicts:
	includes/request.inc
Wrong hook name.
Merge pull request #41 from jordandukart/7.x-USC-11

Make OAI's implementation become generic and overridable for custom handlers.