Added a new mixin psexec.rb

This mixin includes psexec's functionality to execute a single windows command.  It adds the 'psexec' method which takes a single STRING argument which should be a valid Windows command.

For an example of a module using this mixin see the following pull request.
<a href="https://github.com/rapid7/metasploit-framework/pull/1176">1176</a>.