Make derivative generation optional.

By default, derivatives will only be generated if they do not already
exist on the given object. If it does exist, we do nothing.
Merge branch '7.x' of git://github.com/Islandora/islandora_solution_pack_large_image into 7.x

Conflicts:
	includes/derivatives.inc
Merge pull request #55 from adam-vessey/7.x

Make derivative generation optional.