Command line options for --slave and --port

- Includes validation to check that --port is specified along --slave
ResponseExporter improvements to help with result returns in slave

- exportResults now returns the "results" structure in the output json
- IterationRunner emits the above "results" in addition to the exitCode
in the event 'iterationRunnerOver'.
Slave module

- Defines a `Slave` singleton class with only one method `start`
- Calling `Slave.start(port)` will create a web server listening to
`port`
- Accepts POST requests on /run/ for which a new Newman run is
instantiated. The results are returned as JSON.
Start the Slave server when using --slave option.

- All the other options (except for --port) are ignored when using the
--slave option.