Add the git checkout feature

Another feature that I've been playing with is the git checkout. On the terminal when I type "git tree" I see this:

* 7b98e09 (HEAD, origin/git-checkout, git-checkout) Allows the checkout of a git SHA
* 05f31b4 (origin/master, origin/development, master, development) Allows the configuration of any search engine
* 2f52dd7 Remove the hard code current path
* d896e24 Adds the google search feature

So, I thought that it might be cool to make the plugin be capable of do a git checkout based on a sha, branch or tag. 

What do you think?