Added script to upload code reviews

To use the upload script, you run:
$ python upload.py
which will automatically upload all changes output by
"git diff".  If you'd like to upload a different set of changes,
you can use the --rev flag to specify the base revision.
For example, to upload the changes represented by your most
recent commit, run:
$ python upload.py --rev HEAD~1
Or to upload the changes compared to the master branch:
$ python upload.py --rev origin/master