migrate-history.sh: Add -o subdir option

This option allows the usre to specify a subdirectory of the incoming
repository to be imported, instead of importing the whole thing.
Only commits which affect that subdirectory will be migrated.

Testing Done:
The test script passes (added a test for the new option)

Reviewed at https://rbcommons.com/s/twitter/r/715/
Make pex's -i argument compatible with pip's -i argument.  This also allows the use of alternate Python package indexes (e.g. devpi) which use paths other than .../simple.

Since this is a backwards-incompatible change, bump the version number.

Testing Done:
./build-support/bin/ci.sh

Reviewed at https://rbcommons.com/s/twitter/r/737/
RSTify changes and plumb through sdist.

Testing Done:
Manually converted CHANGES with rst2html to validate that it looks sane.
Will publish 0.6.0 to pypi following this, then work on the pex split to
pantsbuild.

Reviewed at https://rbcommons.com/s/twitter/r/749/
Upgrade to pantsbuild.pants 0.0.21

This picks up support for t.c.python 0.6.0 and a broader
range of cheeseshops.

Adapt to package rework in pants apis (goal -> goal.goal, goal.phase)
in the t.c.pants plugins and prep them for a release with a minor
version bump and the start of a CHANGELOG.

Testing Done:
1.) local ci:
==
$ ./build-support/bin/ci.sh

2.) publish test:
==
$ ./pants setup_py pants-plugins/src/python/twitter/common/pants:plugins
Running packager against /home/jsirois/dev/3rdparty/jsirois-twitter-commons/dist/twitter.common.pants-0.4.0
Writing /home/jsirois/dev/3rdparty/jsirois-twitter-commons/dist/twitter.common.pants-0.4.0.tar.gz
$ cd dist/ && tar -xzf twitter.common.pants-0.4.0.tar.gz && cd twitter.common.pants-0.4.0
$ python setup.py --long-description | head
RELEASE HISTORY
===============

0.4.0 (7/25/2014)
------------------

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

* Update to ``pantsbuild.pants`` 0.0.20 api changes

Bugs closed: 313

Reviewed at https://rbcommons.com/s/twitter/r/763/