psexec_command improvements 

There were a couple issues with psexec_command (at least the way I was trying to use it). You couldn't execute things that took a long time to run (it would give an error). Also, large or binary output didn't work.

This pull request addresses these. It waits until the output file is not blocked before downloading output, using configurable retry and delay options. Also it outputs to a logfile by default rather than just to the prompt.