Refactor to make clients simpler and obvious

- Renamed collector module to arguments so that you're importing from
  collector the thing that deals with arguments.
- Moved json serialisation inside Bucket.post so that the client doesn't
  have to deal with it.
- Changed the package name in setup.py, sorry, I was wrong about it.
Add JSON encoder and tests for write module.
Add tests for arguments module.
Add travis config.
Ignore build directories.
Add build status image to readme.
I a typo
Split travis install into two steps.
I another typo
Update readme.
Remove redundant 'from_target' class method

This can be done with Bucket(**target)
Merge pull request #2 from alphagov/refactor-and-comment

Refactor to make clients simpler and obvious