Allow projects to specify additions to the pythonpath.

.dir-locals pony-settings can now inlcude a pythonpath option that
is passed to manage.py using --pythonpath. --pythonpath will only
be passed if it is included in the project.
Merge branch 'custom-pytonpath' of https://github.com/SoftwareMaven/pony-mode
Update authors for Travis Jensen
Update docs to reflect new pythonpath dir-locals code