Added `transpose()` method to MutationAgent.

You can now say:

    >>> staff = Staff("<c' e' g'>4 d'4 <e' g' c''>4")
    >>> mutate(staff).transpose(+6)
    >>> print format(staff)
    \new Staff {
            <gf' bf' df''>4
            af'4
            <bf' df'' gf''>4
    }
Added `get_hash_values()` method to StorageFormatManager.
Improved parameterized __repr__ test.