add priority keyword to `add_system()` method

    Add a `priority` attribute to `ecs.models.Systems` and sort
    `ecs.managers.SystemManager._systems` by it every time a system is added.
    Also add a new test case to ensure that system with the same priority
    end up being executed in the order they were added and adjust
    `test_systems_added` and `test_remove_system`.
Merge pull request #6 from ponderstibbons/master

Add priority keyword to `add_system()` method.