Implement derivatives hook.
Add function comment and get rid of unused variable.
Carry through changes of Islandora constants.
Merge pull request #96 from adam-vessey/7.x-constants

Carry through changes of Islandora constants.
Merge pull request #95 from adam-vessey/7.x-derivatives

Implement derivatives hook.
take care of this outstanding issue via @nigelgbanks comment from https://groups.google.com/forum/#20bookobjects/islandora/8Ptf6ZVHkA4/JlS16B6SMMsJ
Merge pull request #98 from ruebot/7.x

take care of this outstanding issue via @nigelgbanks comment from https:...