Changed PYTHONPATH requirements. Important note included.

Added three new experimental tests.

The tests check that ...

    import blue_example_score
    import green_example_score
    import red_example_score

... all work.

MODEL: the idea is that all score packages should be directly importable in
Python. Even before Abjad is started.

The three example scores are housed in the experimental
tools.scoremanagertools.scorepackages package.

Developers must add the experimental tools/scoremanagertools/scorepackages
directory to their PYTHONPATH environment variable in order for Python to be
able to import the three example scores.

Something like ...

    export PYTHONPATH=$ABJAD/experimental/tools/scoremanagertools/scorepackages:$PYTHONPATH

... should do the trick.

Note that py.test will fail on these three new tests until the PYTHONPATH
environment variable is adjusted.

MODEL: it's recommended that all user scores contain a top-level test/
directory with a test_my_score_import.py test file for exactly this same
purpose. This will help prevent code rot as you build many scores with Abjad.