Add collection_names function to Database

In order to switch the migrations to using our anti-corruption layer
rather than straight pymongo we need to implement the getting of a list
of collections in a db.

We test using is_in because matching exactly will make the test more
brittle.
Switch the migrations to our pymongo wrapper

Our wrapper was making sure that _updated_at was correct in each
document. The migrations not using this wrapper was causing these fields
to be stripped on each run. We now have code that relies on _updated_at
so this was totally unexceptable.
PEP8 compliance changes
Merge pull request #194 from alphagov/fix-migrations

Fix migrations