Add formula for git-review

git-review is a git plugin for handling gerrit-based codereviews. It's an application written in Python, and I tried to follow all the newish conventions on how to write such formulas.
Can a test be added to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!