Add entity modeling and fields

Also define a basic entity for Host
Merge pull request #920 from elyezer/records

Add `Host` entity and underlying models
Create some simple tests for path api/v2/hosts

Implement a few tests which check whether it's possible to GET the path, and add
some partially written tests for doing other things like POSTing and messing
with sub-paths of api/v2/hosts.
Use new `requests` wrapper