WIP-MEI: Fixed: Add sample file to the mei.base module docstring

Previously, this code sample looked like it executed, but it wouldn't. It was
a place-holder. Now it has a real MEI document!
WIP-MEI: Alphabetize imports
WIP-MEI: Prefer music21.ext.six to system six

I'm not sure why I thought this was a good idea in the first place, but it
really wasn't. Fixed it now.
WIP-MEI: makeDuration() uses Duration.dots

Previously, makeDuration() performed the dots calculation by itself. Since
Duration already does this, we can just use that property.
WIP-MEI: Remove unnecessary 'OMIT_FROM_DOCS'
WIP-MEI: Clarify dotFromElement() return value
WIP-MEI: Refactor: noteFromElement() uses Note.__init__() in easier-to-read way

Previously, there were a several function calls in the call to Note.__init__(),
which may have made the code much harder to read in the future. Now, the code
uses more space and variables, but it's easier to understand what's happening.
WIP-MEI: "Synchronize" chordFromElement() with noteFromElement()

Another benefit of the recent noteFromElement() changes is that it can more
closely reflect how chordFromElement() works. Now it's pretty close!
WIP-MEI: Remove embedded calls in restFromElement() and spaceFromElement()
doctests for DOC_ATTR
default for timespanCollection gone
rem _DOC_ATTR in __slots__
fix importlib Python 3.3
doctests in _DOC_ATTR in singleCore
spannerStorage. small dynamic fix
FIX case where reused IDs could result in isDead!
more relevant coverage scores?
rem obsolete code
WIP-MEI: Hook up MEI tests to the usual music21 tests

Previously, running the entire music21 test suite would only run the doctests
from the MEI module. Now, I've added a "Test" class in the "base.py" file, so
that all the "test_base.py" classes will run too.
WIP-MEI: Minor formatting thing