MEI-to-music21 Work-in-Progress Preview

This pull request isn't intended to be merged. As per Myke's request, this is to facilitate line-by-line commentary on the code currently in progress. Thanks for any suggestions and requests you may have!
These print statements need to be gone.  

What's the timing on creating an ETree.ElementTree from a large musicxml file? can't we just look for mei in the first 1000 characters vs musicxml in the first 1000 characters?