Adds Basic Collection Objects

Splits endpoints.json into partials and changes it to import thing.json
and a new collection.json. collection.json contains methods to pull
down a collection from BGG with various flags already set.

Adds Collection response, factory, and entities.

Adds collection fixtures and adds tests for collection classes thus
far.

Creates a FIXTURE_DIR constant in the test bootstrap file and updates
test cases to use that instead of manually figuring out where the
fixtures are located.