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]`
Re-enable tests

This reverts commit 64775270cb224fe1c2a2254dd22b167f14e5b6fd.