Unify API for on-the-fly generated and pre-compiled message classed

[chfritz]

See #12 (comment).

This is complicated slightly by the fact that pre-compiled message classes can be instantiated synchronously, whereas on-the-fly generation of message classes is asynchronous. So a unified interface may need to follow the asynchronous approach, i.e., use a callback rather than a return value.

[chris-smith]

If you promisify on-the-fly message generation (or at least part of it), you could do the asynchronous generation in rosnodejs.init e.g.

rosnodejs.init('/my_node', {messages: ['std_msgs/String'], services: ['std_srvs/SetBool']})
.then(...)

I think the messages should probably be generated before we start checking for the master. So where we create a new promise in the init function, if users provided messages and services in the options, you would instead do

init() {
  ...
  return this.use(options.messages, options.services)
  .then(() => {
    _checkMasterHelper(resolve, 0);
  }
}

[chfritz]

That sounds good!

[chfritz]

OK, PR created.