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/72070902
- https://www.pivotaltracker.com/story/show/72070938

[Finishes #72070902]
[Finishes #72070938]
Add g-cloud collector requirements to setup.py
Move main.py to core.py, put main() in __init__

- Make `__init__.py` the new entry point with a `main(..)` function
- Rename `main.py` to `core.py` a la the other collectors
- Use `performanceplatform.collector.write.[DataSet|JsonEncoder]`
  instead of `backdrop.collector.write.[Bucket|JsonEncoder]`
Merge pull request #24 from alphagov/gcloud-collector

Check in the G-Cloud collector code & README