Removed Measure.__str__() definition.

OLD:

    >>> measure = Measure((3, 8), "c'8 d'8 e'8")
    >>> print str(measure)
    |3/8 c'8 d'8 e'8|

NEW:

    >>> print str(measure)
    Measure((3, 8), "c'8 d'8 e'8")

The observation is that the custom definition of string was unused by any other
part of the system.
Merge branch 'master' of https://github.com/Abjad/abjad