automate release process

Release process now: run script/release that's all to remember. In interactive (default) mode it will guide through bumping version number, editing CHANGELOG, creating release commit and tag, answer `n` to "Upload to PyPI" question.

- script/release will push new tag to Github
- Github will invoke Travis build hook
- Travis will build and upload PyPI package, granted all tests pass

script/release considers these sources of actual version information:

- python3/httplib2/__init__.py:__version__ = 'foobar'
- git tag in -auto mode (on Travis)
- what user entered in interactive mode

Every commit in every branch gets uploaded to Test PyPI, e.g. https://test.pypi.org/project/httplib2/0.11.1.post1803270210/
Every tagged commit in master gets uploaded to Public PyPI.

https://github.com/httplib2/httplib2/pull/99