Changed the MongoDBPurger class to only clear collections

[lavoiesl]

Changed the MongoDBPurger class to only clear collections, not remove them.

Instead of dropping all collections, we should remove all documents in a collection instead. This avoids problems where indexes are lost while dropping the collection, and collections not being recreated (with a schema create tool) while loading fixtures.

Fixes #150 Superseeds #118 from @gerryvdm

[lavoiesl]

I would like to add tests, but nothing is in place to test a Mongo database, so this may take time.

[jwage]

:+1:

[lavoiesl]

Merged to 1.1