WIP-MEI: New: Add music21.converter.subConverters.ConverterMEI

Required for the music21.converter module to automatically parse MEI files.
WIP-MEI: New: UNTESTED music21.converter.__init__ Imports MEI

Couldn't figure out proper testing yet, but it appears to work...
WIP-MEI: Fixed: Add "mei" Module to music21.__init__
WIP-MEI: New: Add <rest> Importing and Tests
WIP-MEI: Fixed: Minor Update to noteFromElement() Docs

Just to bring the "status" message up to date with reality.
WIP-MEI: Fixed: Descriptions of <note> Tests Reflect Their Purpose
WIP-MEI: New: Preliminary Implementation of mei.__main__.convertFromString()

This function will require extensive work as more elements are implemented, but
this works for now.