Refactor the three proxying APIs into methods of a single network object.  Introduce an in-memory implementation of that interface to ease authorship of tests that depend on this layer.
A few doc followups.
Move Proxy to a model module.
Factor the common test helper into its own module.  Move the in-memory tests to their own test module.
Add some model tests for Proxy.
Fix PEP8 whitespace error.
Merge pull request #206 from ClusterHQ/testable-routing-188

Introduce an in-memory test double for the proxy configuration APIs.