Add pep425 tagging support to twitter.common.python

This adds PEP425 tag iteration to twitter.common.python:
http://legacy.python.org/dev/peps/pep-0425/

Wheel platform tags are encoded in this fashion and is the standardized way
to determine whether or not a wheel distribution is compatible with the
current platform.

Reviewed at https://rbcommons.com/s/twitter/r/87/
factor all twitter.common.python test helpers into twitter.common.python.testing

this is part of the effort to pull all python_library targets out of tests/python

Reviewed at https://rbcommons.com/s/twitter/r/91/
doc: want to publish an artifact? publish ALL the things. maybe. many, anyhow
comment: publishing the docs? use ssh url, it's way smoother
Add RuntimeError if we fail to write pkg_resources into the .pex

Reviewed at https://rbcommons.com/s/twitter/r/115/
Merge branch 'master' of github.com:twitter/commons into jsirois/pants/multiplatform_bin