Allow pomegranate:reindex Rake task to be used in production

[escowles]

• Move the pomegranate:reindex task out of the dev.rake which is only loaded in dev/test environments
• Use IIIFResource.find_or_initialize_by instead of find_by to allow initial indexing with the Rake task
• Allow finding the resource by noid, for items that have already been indexed once
• add a task that commits the index

[hackartisan]

seems like it would be nice to have a task that takes a noid, as well as one that could do an initial index.

[escowles]

Also, as long as we're updating our indexing Rake tasks, it would be good to have a task that commits the index, since we've disabled commits in our code, and autocommits take a long time.