WIP-MEI: New: (Untested) Function for <dot>
WIP-MEI: New: (Untested) Function for <accid>
WIP-MEI: New: (Untested) Function for <artic>
WIP-MEI: Fixed: Change music21 Version of "natural" to "n"

We used to translate @accid="n" to "" because I thought specifying "n" to
Pitch.__init__() would break it. But translating "n" to "n" streamlines
things.
WIP-MEI: Fixed: Improved <beam> Type Specification

We used to use duration.convertQuarterLengthToType() to get the type of
a beam, but this doesn't work for dotted duration. On the other hand,
duration.dottedMatch() does.
updates to installation docs.
remove test that rebuilds core corpus
add runTime to multiprocessTest
WIP-MEI: New: <note> can have <artic>, <dot>, and <accid> inside

And it's tested, too!
WIP-MEI: Formatting: Fixed some things, suggested by pylint
test new multiprocessTest
Merge remote-tracking branch 'upstream/master' into mei