Add pagination capability to API

[dlongley]

We need to support getting partial views of collections (e.g. to support pagination of messages on a frontend) with this module. We could try and model the message collection as an LDPC and see how that goes. See:

https://dvcs.w3.org/hg/ldpwg/raw-file/default/ldp-primer/ldp-primer.html
https://www.w3.org/TR/ldp/