add possibility to bind to an host

[RomainLanz]

Implement feature #4

I have also make this spec more consistent.

Alternative

Server.prototype.start = function (port, host) {
  host = host || '0.0.0.0'
  this.Logger.info('serving app on port %s', port)
  this.serverInstance.listen(port, host)
}

If we implement the alternative we don't need to change tests.

Mirror PR: adonisjs/adonis-app#1

[houndci-bot]

Missing semicolon.