Add revision support to get cli command

I was playing around with the doozer client as a way of getting a feel for solving some problems with Doozer, and was surprised that get did not return the revision. I added this feature to the client. Since that was so easy, I also made another commit to allow passing a revision to get. You might not want the second commit in it's current form though - I wasn't sure how to make the revision argument optional. Cheers!