Capturing Puppet agent output

Hi Simon,

Without this change, I wasn't seeing any messages that were appearing on stderr.  I've changed the code to more closely follow the example at http://docs.puppetlabs.com/mcollective/simplerpc/agents.html#running-shell-commands and that appears to have fixed the issue for me at least.

I now see no output on successful agent runs, and get the agent error messages back if there were any failures.

Cheers,

Matt.