Adds a run command for running one-off processes

* nf.js run command exits the main process with the same code as the child
process
* proc.js once function passes the exit code to the callback function
* test counts successful callbacks and calls assertions on process exit
Merge pull request #54

Closes #54
Exit with non-zero on fatal error

 * Preventing export from finishign is a fatal error
 * Missing arguments is a fatal error

Closes #60
Fix proxy errors on Windows

Check for setuid/getuid as means of platform detection before checking
if we need to setuid to SUDO_USER.

Fixes #57
Closes #58
Propagate signals on exit

When killing workers due to a signal, propagate that signal by using it
to kill the other workers.

Inspired by #52
1.1.0

 * Propagate signals on exit (Ryan Graham)

 * Fix proxy errors on Windows (Ryan Graham)

 * Exit with non-zero on fatal error (Ryan Graham)

 * Adds a run command for running one-off processes (Jeff Jewiss)

 * Add contribution guidelines (Ryan Graham)