oai-pmh: add custom HTTParty client

Motivation:

Currently, there is a lot of HTTP/networking code mixed in with
application code.  It would be good to seperate these concerns,
particularly as we want to introduce connection reuse.

Modification:

Introduce new class that uses HTTParty as a mixin.  Update code to take
advantage of this new class.

Result:

Reduced code duplication; now easier to add new features.