Travis config to deploy to testpypi

[abadger]

Implementation of the Pypi portion of #70. This commit uses the test.pypi.org server rather than the real server. When we're satisfied that this does the right thing we'll need to switch over.

What's left to do:

• Create a twiggy-uploader user on the real https://pypi.org/
• Grant maintainer status to twiggy-uploader on pypi.python.org so that it can upload to pypi.
• Add the twiggy-uploader password to the .travis.yml file
  • Use travis encrypt --add deploy.password to encrypt the new password

Release procedure:

• Make final changes for the release
• git push
• Check that the commit passes in travis
• git tag $RELEASE
• git push --tags

Note that travis will upload to pypi as soon as the first test passes in travis. So you don't want to push the tags until travis has run over the commit once and shows that the build passed (in case one test passes but other tests fail)

[abadger]

The test instance of pypi that this PR pushes the new releases to is here:

• https://test.pypi.org/project/Twiggy/
• https://testpypi.python.org/pypi/Twiggy

[abadger]

This PR will build both sdists and wheels for upload to pypi.

[wearpants]

Ok, I created a twiggy-uploader user on the production PYPI and set a password for it. I wasn't quite clear on the order of operations, so I didn't add it to PR, here's the encrypted value:

lwGo2bGY8NHj96K+l0jzUiJX2+ZUs/DACwZt94VXKmOTOeqU1sPOBunh4ajSGUJCo2KSG17QasiHEBNPQKnA3DjKm76v4DdpNKwHxzsENyy2B4QicJa/qur3OzpY0ozW0oei0Nj7oFydrfFZoY6NBrt0ox4bjJcQHoj+S4XmNKk=