Cleaned up CodeBlock doctest.
Improved tox.ini keyword argument handling.

You can now pass keywords arguments to each py.test session run by tox like so:

    abjadroot$ tox -- -mypytestkeyword -myotherkeyword

Note the double-dash preceding the keyword definitions. Those characters act as
a delimiter between the keywords passed directly to tox and the keywords passed
through to the tests run *by* tox.
Merge branch 'master' of https://github.com/Abjad/abjad