Bundle a newer version of Python with the Windows RBTools installer.
Bundle a newer version of Python with the Windows RBTools installer.

Our Windows installer for RBTools was dependent on PortablePython, which
is no longer maintained. It was pretty annoying even using
PortablePython, given how many packages it wanted to ship and the
bootstrapping process we needed for the initial extraction.

Turned out that the only parts of PortablePython we cared about were the
wrapper around installing the Python MSI, which was easy enough for us
to do. We now incorporate that logic plus some additional logic to
update pip/easy_install. This makes our installer simpler and easier to
maintain, particularly when new Python versions come out.

The new installer has been bumped to ship Python 2.7.12.

Testing Done:
Built the installer from a clean environment. Installed the resulting
installer on a separate Windows environment. I was able to successfully
use all RBTools commands that I tested.

Reviewed at https://reviews.reviewboard.org/r/8575/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools