add unix reverse python shell

Split up #1371 as per https://github.com/rapid7/metasploit-framework/blob/master/CONTRIBUTING.md

This python shell was originally submitted a long time ago as part of the reverse SSL pull requests, since another reverse_python shell exists in framework, i've commented on the differences and appended a 2 at the end of the module name. Feel free to rename as needed if included (reverse_python_loop) or something to that effect.