Implement pagination parsing for collection responses (ref #76)

This implementation follows the structure outlined in the discussion of #76:
parse the header information, and use it to augment the response collection.
The presence or absence of `._paging` can therefore be used to quickly
assess whether the response collection is complete, or just one page's worth.

I ended up not taking the approach of implementing this as a drop-in method
to mutate the response data, as I needed pagination handling now and we
have more discussion to do before we go forward with the response collection
augmentation interface. There was also a question of whether that interface
would be exposed pre-returnBody, or after it; if we went with the latter,
there would be no way to access header information.

We've opened a conversation with @rmccue about how Pagination may evolve
within the ReST API plugin; this solution will evolve along with the plugin
itself, at least up until the API hits core.