Querying for multiple components at once

    EntityManager.{components_for_entity,pairs_for_type} now take a variable
    amount of component types. When only one type is given they should behave
    exactly as the old version and if not the single component instance is
    replaced by a tuple of component instances in the order of the types given.

Since I have some free time at hand at the moment, I just went ahead and implemented this one.