Made cross-version compatibility changes in musicexpressiontools.

All musicexpressiontools tests and doctests pass both versions.
Made cross-version compatibility changes in newabjadbooktools.

All newabjadbooktools tests and doctests pass under 2.7 and 3.4.
Made remaining cross-version compatibility changes in experimental.

All experimental tests and doctests now pass under 2.7 and 3.4.
Merged master.