Initializeable

For issue #24 
I agree that this method will work. However, this makes things a bit ugly. Earlier, things were cleaner as most bricks subclass Brick, and I can imagine an intuitive hierarchy of inheritance. Initializeable is a mixin, which provides a specific functionality. Making it part of the class hierarchy does not seem like a good idea, and it will make it harder for beginners to understand the framework.

There could be other such but orthogonal functionality, say Regularizeable. If both Regularizeable and Initializeable were mixins, we have no problems. But if we make Initializeable a part of the class hierarchy, then so would be Regularizeable, and then we would be confused which to subclass.

I think that the reason _push_initialization_config and push_initialization_config both belong not the base class, but to the Initializeable mixin class, since both functions deal with initializations.

What do you guys think?