Better command line call, namespace, and existing file fixed

Hi there, 
I've done those changes in my branch and I would like them to be merged so I can get rid of my fork. 

I've changed the way locale is passed on the command line becasue I was getting a kind of error from rake. Passing that like an ENV var seems more clean to me. 

I also added namespace for generated file, it gets loaded the same, and chances that one has an en.rb already are not so rare.