Deprecate multiple init methods in favor of registration methods

@arrouse @sixstringtheory 

As proposed in #7 this gets rid of the initializer methods in favor of just using the registration methods. There was significant redundancy there since the init methods were simply calling the public registration methods anyway.

Also changed:
- Add appledoc-style comments to all public methods
- Add assertions for required parameters
- Modify @sixstringtheory's nib name implementation (#6) to allow multiple custom nib names per manager
- Remove unintentional usage of the UIView category method for `setFrameWidth:`