super() rootpy

This PR converts all classes over to the use of Python's `super()` to handle calling through the MRO.

In the process of overhauling the code, a few other items were addressed:

* Non-rootpy subclassed objects can be included in Legends.
* `File.Get` and `__getattr__` return fully rootpy-ized Directories in a consistent way.
* Plottable initing (decoration) is fixed (not all attrs were inited previously)
* Some `__init__` code has been simplified (thanks to `super()`).