presubmit_support.py can now creates a Rietveld object on its own.

This is necessary since the object needs to be recreated out of process. The
commit queue runs the presubmit check out of process for sanity reasons.

Added TODO to push the cookie instead of the password.

R=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7058054

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@87842 0039d316-1c4b-4281-b951-d872f2087c98