WIP-MEI: add 'mock' to the music21.ext, on Myke's request
WIP-MEI: Fallback to music21.ext.mock if required
WIP-MEI: Update licences
WIP-MEI: Rename __main__ to base and test_main to test_base
WIP-MEI: changes for previous commit's renaming
WIP-MEI: Minor correction of terminology
WIP-MEI: Create an 'MeiToM21Converter' class, as per Myke's suggestion

So far, the class is merely a wrapper for mei.base.convertFromString(). The
plan is that the class will hold shared state among all the converer functions,
avoiding the need to pass around things like the "slurBundle," for example. It
may also help in the effort to simplify convertFromString(), which will
eventually disappear into this class anyway.
WIP-MEI: Write and Test MeiToM21Converter.__init__()

The initializer handles a rough document validity check, so that's nice.
WIP-MEI: Move convertFromString() into MeiToM21Converter.run()

So far this is simply a copy-and-paste movement. The following commits will
address the optimizations made possible with this change. Following that,
I'll continue my crusade to simplify the method that is now run().
WIP-MEI: MeiToM21Converter.__init__() does more stuff

The class now accepts zero-argument initialization so it's pickle-able (and
easier to use in testing).

The __init__() also initializes the "slur bundle" and "m21Attr" instance
attributes, because initialization is what __init__() is for.
WIP-MEI: base._ppSlurs() now uses MeiToM21Converter

Rather than many parameters, _ppSlurs() now uses the shared state object.
WIP-MEI: base._ppTies() now uses MeiToM21Converter

Rather than a bunch of stuff, it's using just the one.
WIP-MEI: base._ppBeams() now uses MeiToM21Converter

And somehow this is better because, uh, I explained in previous commits.
WIP-MEI: base._ppTuplets() now uses MeiToM21Converter

Read previous, similarly-named commits for more info.
WIP-MEI: _ppConclude() now uses MeiToM21Converter (plus tests!)

This function wasn't previously tested, but now it is.
WIP-MEI: Rewrite makeDuration() docstring and tests

As requested by Myke, I converted the automated tests into doctests, which
helps to clarify the method's use.
WIP-MEI: Add doctests for _accidentalFromAttr() and _qlDurationFromAttr()

These amount to examples for the docstrings. As requested by Myke.
WIP-MEI: Improved docs for consistency and readability
WIP-MEI: allPartsPresent() has better documentation; uses MeiToM21Converter

Previously it wasn't clear what this function does. It's improved now, including
a nice doctest as an example.

The function also now uses the shared object, and runs the XPath query
internally, so it'll be easier to improve the function in the future.
WIP-MEI: Revised the _ppSlurs() (etc.) docstrings

I added an example to _ppSlurs() but not to the others. Considering these
functions are not intended for use outside the module, and anyone revising
them will need to seriously thing about what they're doing, I hope this
level of documentation is sufficient.