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.
Changed Parameter Type Hinting to only use the top level Abstract class
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.
Change mimetype back.

"image/tiff" is correct... It's just our Mimedetect class which is/was
returning the incorrect mimetype.
Merge pull request #63 from adam-vessey/7.x-fix-mime

Change mimetype back.
Merge branch '7.x-fcrepo4' of git://github.com/nigelgbanks/islandora_solution_pack_large_image into 7.x-nigel-merge

Conflicts:
	includes/derivatives.inc
Merge pull request #64 from jonathangreen/7.x-nigel-merge

7.x nigel merge