Release 0.0.20 with updated twitter.common.python dep.

This also adds a CHANGELOG.rst and incorporates it in
releases via the setup.py long_description field. Release
docs are updated to point out this file needs to be edited
along with the version.py when prepping a release.

Testing Done:
1) verify the long_description addition is going to work:
==
$ PANTS_DEV=1 ./pants setup_py src/python/pants:pants-packaged
*** Running pants in dev mode from /home/jsirois/dev/3rdparty/jsirois-pants/src/python/pants/bin/pants_exe.py ***
WARNING!  /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants.YHc2bE/src/pants/docs/conf.py is source but does not belong to a package!
WARNING!  /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants.YHc2bE/src/pants/docs/pagerank.py is source but does not belong to a package!
Running packager against /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants-0.0.20
Writing /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants-0.0.20.tar.gz
$ cd dist/ && tar -xzf pantsbuild.pants-0.0.20.tar.gz && cd pantsbuild.pants-0.0.20

# The concat of ABOUT.rst and CHANGELOG.rst is effective:
$ python setup.py --long-description
Pants is an Apache2 licensed build tool written in Python.

The latest documentation can be found `here <http://pantsbuild.github.io/>`_.

RELEASE HISTORY
===============

0.0.20 (7/25/2014)
------------------

API Changes
~~~~~~~~~~~

* Hide stack traces by default
  `Issue #326 <https://github.com/pantsbuild/pants/issues/326>`_
  `RB #655 <https://rbcommons.com/s/twitter/r/655/>`_
...

2) burn the new release docs: http://pantsbuild.github.io/jsirois/release.html

Bugs closed: 396

Reviewed at https://rbcommons.com/s/twitter/r/754/