Document the test-app-path option in test-packages

As using it is the only way to run tests on iOS
Rework --mobile-server defaults.

* --port now requires a port ('meteor run --port example.com' isn't valid).
* --mobile-server defaults to your detected IP address and the port from
    --port.
* If you provide a value for --mobile-server, we default to http:// as
  the protocol. A host is required for --mobile-server if you don't omit
  the option entirely. Similar for the --server argument to 'meteor
  build'.

This commit includes the 'netroute' npm module as a core package (which
has binary dependencies) for IP detection. It would be nice to put it in
packages/non-core, but I think it has to be a core package in order to
uniload it.