Add DataSet json endpoints and tests

As the provider of config data, Stagecraft needs to expose this information so that Backdrop can read it's configuration.
Fix tests on Python 3.3
PEP8
Return 400 instead of 404 on malformed dataset URL

We should return a Bad Request response, not a Not Found response, when a bad query string key is requested.
Improving error JSON in data-set views
Merge pull request #24 from alphagov/endpoint-for-backdrop

Add DataSet json endpoints and tests