added autocompletion env script

I believe this is the correct way to add the env script to a module. I could have used a static file instead of a template, and there is possibly something I am not thinking of at the moment.

Please let me know if there is another/better way.

This should resolve issue #1 


I believe we have this path in some puppet fact and we can use here to not use the `$BOXEN_HOME` environment variable. Could you take a look?