WIP-MEI: Move placement of 'mei' in music21/__init__.py

I put the 'mei' module in the list of individual files, rather than the list of
directories. Fixed it now.
WIP-MEI: Raise error when exporting 2048th duration to MusicXML

Although it works in MEI, 2048th notes are not supported in MusicXML. Now,
music21 will raise an error when someone tries to export a 2048th duration
to MusicXML.
WIP-MEI: Improve "mei" module docstring

I forgot I still had to do this, so thanks to Myke for catching it!