handle failed commands, return actual exit code. fixes #4
Fix for running on windows
Add test case for non-zero exit code
Move to using a node worker to handle exit codes and stdio
Add "output" property and fix tests
Remove failing test case
Add retries to the rimraf code

It will still give up after 200ms
Remove race condition in tests
Merge pull request #7 from ForbesLindesay/handle-failed-commands

handle failed commands, return actual exit code. fixes #4