Release v2.0.1.

[dhermes]

Made the release notes by checking out the commits since the last release

$ git log -1 --pretty=%H
4b0a5edd47142a02e5bf585d8009195a29036449
$ git log v2.0.0-post1..HEAD \
> | grep 'Merge pull request ' \
> | awk -F ' ' '{print $4}' \
> | sort

yields
#416
#419
#421
#424
#425
#426
#429
#430
#431
#432
#434
#437
#438

[dhermes]

Commits left out of release notes:

• GCE only system test #416: Adding GCE system test (to prevent future breakages)
• Fix GCE system tests #425: Removing a brittle part of GCE system test
• Removing all uses of self.fail(). #430: Removing TestCase.fail() usage in unit tests
• Getting to 100% line coverage in tests. #431: Fixing unused branches in unit tests
• Using mock App Engine module when building docs. #432: Fixing GAE breakage in docs build
• correct p12 example code typo in CHANGELOG.md #437: Fixing typo in example in change log