WIP-MEI: Tests: Fixed the _processEmbeddedElements() Test

Previously, it had the wrong expected error message.
WIP-MEI: Tests: Add tests for beamTogether()

Following the discussion on MEI-L (link below), I've realized the function
already works as required. So here are some tests.

https://lists.uni-paderborn.de/pipermail/mei-l/2014/001337.html
WIP-MEI: New: <beamSpan> elements handled in convertFromString()

It'll still need to be modularized and tested, but it seems to work!
WIP-MEI: Add "note" to list of things embedded in a <chord>

This is just so _processEmbeddedElements() doesn't report an unprocessed
<note>, which may reasonably worry users. The chordFromElement() function
does process <note> elements, but in a different way.
WIP-MEI: Removed "mock" import in music21.converter.__init__

I don't remember why I put that there, especially when there were no tests using
it. It prevented "from music21 import *" from working unless you had installed
the mock package---even if you weren't going to run the tests!