Test double for code that uses txTwitter

Code that uses txTwitter needs to be easily testable. To this end, we should provide a fake implementation that can be used in such tests.

The current plan is to build a simple fake backend that can be populated with data and responds the same way the real Twitter API would, within certain parameters.

This is a fairly large chunk of work, but an initial implementation will provide value in the short term and it can be fleshed out and expanded later.