New tool to dump 'buckets' collection out to JSON

Just dump it exactly as it is in the database. There needs to be a 2nd
stage in stagecraft to find data_group and data_type by name and create
models if necessary.

See https://www.pivotaltracker.com/story/show/66132542

[#66132542]
Merge pull request #248 from alphagov/dump-buckets-tool

New tool to dump 'buckets' collection out to JSON