Corrections for byte literals, spacing, minor function change, and up…

…dated the examples. (worawit#22)

* Force byte literals where required and corrected the string.ascii_letters(). Also corrected
the spacing to conform to the Python style guide (4 spaces and not tabs).

* Force byte literals where required and removed all trailing whitespace. Added
service_exec() from https://www.exploit-db.com/exploits/42315 and updated the
examples in do_system_mysmb_session(). Confirmed to successfully exploit
Windows XP.

* Uncomment the RemoteShell() as it is confirmed to work by the upstream author.