add a class to convert from absolute MIDI tick to milliseconds

[shimpe]

same as before, but now on the python3 branch

it seems to work for me, maybe it's also interesting for you

I've implemented it as a wrapper class to avoid messing up the existing code base and to avoid making the system slower if this functionality is not needed.

[shimpe]

test failures are probably a travis-ci setup issue?

[coveralls]

Changes Unknown when pulling fd49eb630d65954950ac0e940a62fc0323571fa6 on shimpe:feature/python3 into ** on vishnubob:feature/python3**.