Add Actor subclass to entity's __init__.py

[pushfoo]

It should do the following:

• [x] subclass NamedAnimationsSprite from CatBurglar/entity/__init__.py
• [x] use the same keyword arguments as its parent class
• [x] have some support for moving based on velocity / change_x currently set in the update method so we can use super
• [x] consider what minimal physics method would be most convenient but also lightest for this. we just need walls for now if there's a clear answer, use it.