Add rna.clients

Tests for RestClient base class are complete, but still need tests for
the rest of the client classes.

Also TODO: docs, especially docstrings, flake8 compliance
Accept modified kwarg in RestModelClient.model and models

Default to False, pass through to serializer.save_object
Change UnmodifiedTimestampSerializer.save_object

Accept modified kwarg and only set the default to false instead of
unconditionally setting it to False
Merge branch 'clients' of github.com:jgmize/rna into clients

Conflicts:
	rna/clients.py