Update the release docs and unify the pants semver.

This centralizes the pants version editing to
src/python/pants/version.py and firms up a manual release
process that will be dogfooded after this commit.

Outstanding work includes automation of release testing
and a scheme for editing & publishing a CHANGELOG.

Testing Done:
Ran the release test command block and that works - tail lines included in the docs itself.
Also burned a test site, see: http://pantsbuild.github.io/jsirois/release.html

Bugs closed: 357

Reviewed at https://rbcommons.com/s/twitter/r/690/