Add paged datastream plugin

[ctgraham]

JIRA Ticket: N/A

What does this Pull Request do?

Adds a plugin which allows for the bagging of pages of paged objects with the parent object's bag.

What's new?

New plugin plugin_object_ds_paged which, for select content types, will leverage islandora_paged_content to find and add the datastreams for pages within the parent object's bag.

How should this be tested?

A description of what steps someone could take to:

• Enable plugin plugin_object_ds_paged in the module's settings
• Bag a object without pages (e.g. findingAidCModel, largeImageCModel, etc), and no change occurs
• Bag an object with pages (e.g. manuscriptCModel, bookCModel, newspaperIssueCModel), and the datastreams of the paged objects will be added to the bag's data, under folders named by the page object.

Additional Notes:

• This plugin depends on islandora_paged_content, but should fail gracefully if not present.
• Directory and datastream naming is modeled off of plugin_object_ds_basic

Interested parties

@Islandora/7-x-1-x-committers

[mjordan]

@ctgraham very good, I'll review and test this.