Add Connection#_database
Add DatabaseLike#_collection
Add SystemDatabase#_createCollection
Override Json#toString to pretty print JSON for easier debugging
Add Disposable#using
Add ScarangoSpec and clean up test code
Remove unnecessary imports from test code
Remove mistakenly commented out code
Change Executor#client from def to lazy val

Further clean-up of Executor is planned with another branch.
Merge pull request #2 from sumito3478/priv/test-cleanup

Clean-up test code and add some functionality to do that