Expose cymunk layers + offset for BoxShape

[SleepProgger]

This PR mainly exposes the cymunk layers and the offset for BoxShapes (kivy/cymunk#49).
This PR will break BoxShapes for user with old cymunk versions (before the above PR) !
It might be wise to split this PR into two ?

To make using the layers (a bitset) a bit easier i added the LayerHelper class which allows mapping of layer names to bits.
The shape_info dict may now contain a layers key which holds an iterable of registered layer names or an int representing the bitset.
I am not sure if the LayerHelper should live in kivent or cymunk.

Other changes:

• Made all offsets optional. Defaulting to (0,0)
• Removed outdated info from init_components doc string.

[SleepProgger]

Sorry, missed some debug prints i left in the code.

Additionally i forgot to mention that the CymunkPhysics.init_component functions now raises an ValueError if the shape_type is invalid.

Also, i am wondering: Is PhysicsComponent.shape_type used for anything at all ?
I searched and didn't found anything where its used.
I'd propose to a.) remove it or b.) Remove the _shape_type variable but keep the getter property which could simply return the type of the first shape. I don't see for what tho tbh.