Fix humdrum link
Merge pull request #61 from jamesdoherty/master

Fix humdrum link in docs
Merge remote-tracking branch 'upstream/master' into mei
WIP-MEI: New: Importing <clef> Objects
WIP-MEI: Docs: Update the Implememnted/Not-Implemented Settings
WIP-MEI: Tests: better use of MagicMock's spec_set in mei/test_main.py
WIP-MEI: New: Importing <layer> to stream.Voice