Force ecto

[riverrun]

Moved the database functions to within the library. This has the following effects:

• the openmaize_ecto.ex and openmaize_ecto_test.exs files can be deleted as they are no longer used
• the db_module keyword argument used in many of the plugs has been replaced with a repo and user_model argument
  • the defaults are MyApp.Repo and MyApp.User, so this will hardly affect anyone
• Ecto is a hard dependency - it is no longer optional

Closes #67