Added cleanup method to Psexec-Mixin

I think it makes more sense to have the cleanup_after method part of the psexec mixin.  This way other modules like psexec_command, psexec_ntdsgrab, and other modules that I have written and will be submitting don't have to copy and paste.  I tested it out and it works great.  If you approve I will submit pull requests to those two modules with the appropriate updates.

Thanks!