Added a rev_ssh.py module, changed default encoder back to shikata_ga_nai

This PR adds a rev_ssh.py module which is a port of https://github.com/trustedsec/meterssh for veil-evasion,
for the payload to compile I had to install the Paramiko and ecdsa python libraries.
Additionally I changed the default encoder for msfvenom to shikata_ga_nai, don't know if that might break any payloads though.