Adds Powershell Technique to current_user_psexec

Using this technique means the user does not have to have an accessible share, and no binary is dropped.

There appears to be an issue with exploits that receive multiple sessions due to a threading deadlock issue:

http://dev.metasploit.com/redmine/issues/8407

This has also been observed in https://github.com/rapid7/metasploit-framework/pull/1732