Adding COLLECTION_POLICY Support

[abbistani]

I have a first stab at adding COLLECTION_POLICY support here: https://github.com/abbistani/islandora_collection_csv_import/tree/collection_policy

The advantage is that it works nicely, and was easy to write. The disadvantage is that it requires access to the server filesystem, and a certain comfort level with editing the policy XML files. I plan on extending it and supporting an alternate approach, where instead of providing a single COLLECTION_POLICY column with file paths, users could provide two columns, DEFAULT_NAMESPACE and CONTENT_MODELS, and the module would generate the COLLECTION_POLICY file for them.

Does this approach sound good? And would you like a PR with the current version? We're currently using it right now, and it seems to be working well.