This PR replaces #45 in order to clean up its history.

This addresses #43. It moves plotting-function enhancements from dedicated "wrapper" functions to the methods themselves, breaks apart some really unwieldy wrappers into their component parts, and (just like matplotlib) combines the keyword arg tables associated with each plotting wrapper using matplotlib.docstring.dedent_interpd.

• 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 also 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 matplotlibdocstrings 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.