Replaced importlib.import_module() with __import__().

importlib is only available in Python 2.7.


git-svn-id: http://music21.googlecode.com/svn/trunk@3386 5c16147e-a200-11de-b0cc-ab0ad15e60d0
Fixed spelling in warning message.

git-svn-id: http://music21.googlecode.com/svn/trunk@3387 5c16147e-a200-11de-b0cc-ab0ad15e60d0