WIP-MEI: Preprocessing functions no longer return "documentRoot"

They would never and should never modify the element tree anyway, so there was
no reason to return it.
WIP-MEI: Tests: Add test for _ppTies() (tie preprocessing)

It's a pretty simple test, but hopefully it can be the template for the more
complicated preprocessing methods.
WIP-MEI: Tests: Add test for _ppSlur() (slur preprocessing)
stupid py2 coding breaking py3 on stream
Merge branch 'master' into mei