Review 130828 clean up commands

I've made 2 changes.

1. Use [GLI](http://davetron5000.github.io/gli/) to manage all git-review commands. This helps to save a lot of troubles with checking and parsing arguments.

2. Before creating a new pull request to upstream, check if there already exists one for the current branch. If so, it will not proceed and ask the user to update the request, as discussion in #48.