Allow to specify port

[MattiSG]

PORT=4800 npm start

> sgmap.ludwig@0.3.0 start /Users/sgmap/Dev/ludwig
> babel-node server.js

Express server listening on localhost:3000, in development mode

The PORT env variable is a very common convention. However, since most of the setup of this app is made through npm config, we should support port in the npm config (which will also make explicit the default port), overridable by env.PORT.

[MattiSG]

I see you're doing:

var env = app.get('env');
var config = {
    port:env['NODE_PORT'] || 3000,
    ip:env['NODE_IP'] || 'localhost'
};

But:

1. NODE_PORT is very uncommon.
2. You're mistaking app.get('env') with process.env. The former is more or less an equivalent of NODE_ENV and contains stuff like development or production, not the list of environment variables 😉