WIP-MEI: Fixed: Used @clefshape instead of @shape in <clef>. Now fixed.
WIP-MEI: mei.__main__.allPartsPresent returns tuple instead of list
WIP-MEI: New: <mRest> Importing

For importing, we'll handle <mRest> as a wrapper around <rest>.
Also added <mRest> to layerFromElement().
WIP-MEI: New (Untested): Importing <beam> Tags
WIP-MEI: New (Untested) Function to Import <scoreDef>
WIP-MEI: (Untested) Significant Revisions to convertFromString()
WIP-MEI: Some "Random" Stuff