This PR replaces #111 in order to address major structural changes to the repo file system.

This addresses #43. I got the ball rolling but it probably won't be finished until I have a complete week or so to dedicate to this. It moves plotting-function enhancements from dedicated "wrapper" functions to the methods themselves, breaks apart some really unwieldy wrappers into their component parts, and combines the parameter tables associated with each plotting feature into single docstrings.

• This will make development and debugging much easier. The plotting overrides are currently the source of a majority of bugs reported by users, in part because the current implementation was thrown together in piecemeal fashion and (while I do my best to maintain it) is held together with duct tape...
• This will make it much easier for new users to learn ProPlot -- people have a hard time wrapping their heads around wrapper functions ;)
• This will make the tracebacks much cleaner. It can be pretty disorienting when an error is raised and there are 3x more frames than necessary.

When using proplot locally, the proplot and matplotlib docstrings will be concatenated with nice headers for each section using _concatenate_docstrings in axes.py. When sphinx is importing proplot for generating documentation, they will not be concatenated -- only the proplot docstring will be shown with an intersphinx link to the matplotlib one. This is done by adding docstring.hardcopy: True to the local .proplotrc. Then _concatenate_docstrings skips concatenation if rcParams['docstring.hardcopy'] is True.