Check in the G-Cloud collector code & README

Note that this is just to get the code into alphagov's github - it's not
yet integrated as a performanceplatform-collector.

- Add code, sample data and tests
- Add README which explains what's going on

See:
- https://www.pivotaltracker.com/story/show/70840170
- https://www.pivotaltracker.com/story/show/72070938

[Finishes #70840170]
[Finishes #72070938]