Implement promise interface for non-stream-based commands

[sam3d]

This PR implements a simple optional promise interface as outlined in #33. Luckily based on what you guys said, it doesn't break backwards-compatability (🎉) as these methods didn't return anything anyway. You can now use the non-streaming commands in the following ways:

Standard

serf.members((err, members) => {
    console.log(members);
});

With promises method

serf.members()
.then(members => {
    console.log(members);
})
.catch(err => {
    throw err;
});

With await/async

let members = await serf.members();

I've also included some brief documentation in the README, but I've had to include b6dd867 because I couldn't get it to work on my machine without including it. Once the other #31 has been merged I'll rebase this with master and remove that commit so it doesn't clog the log, so to speak.

[sam3d]

Ah my workflow now looks like this:

let port = 5918;

let agent = await serf.agent(port);
let client = await serf.client(port);
let members = await client.members();

console.log(members);

And I can't tell you how happy that makes me that it looks this simple 😄

[zbjornson]

Code looks good 👍. Would be nice to add a test for one of the methods.

[zbjornson]

Minor typo: s/them/the

[sam3d]

Oops my bad, will fix that now and add a test 👍

[sam3d]

Wait no the tests are failing, I'll give that a look-over bear with me.

[zbjornson]

Hm, actually I wonder if it should only return a Promise if there is no callback provided. It's unfortunate to get UnhandledPromiseRejectionWarnings if you're not using promises -- especially since you're probably handling the err argument in the callback.

[sam3d]

Good plan, let's give that one a try.

[sam3d]

Okay so this is odd - the particular test case now passes but it breaks all future test cases:

Serf stream works with a "listen" callback:
 Uncaught Error: This socket has been ended by the other party
      at Serf.writeAfterFIN [as write] (net.js:290:12)
      at dowrite (serf.js:204:10)
      at Serf.send (serf.js:188:5)

[sam3d]

Okay after testing locally this happens too, I hadn't tested multiple access types. For some reason when attempting to make another request after the first promise-based one, none of the other requests work. Will look into this now.

[zbjornson]

You can monitor the serf process's output with something like

+    procs[0].stdout.on('data', d => console.log(d.toString()))
+    procs[1].stdout.on('data', d => console.log(d.toString()))
+    procs[0].stderr.on('data', d => console.log(d.toString()))
+    procs[1].stderr.on('data', d => console.log(d.toString()))

and get...

2017/10/26 21:47:11 [ERR] agent.ipc: Failed to evaluate request: command '' not recognized

I'm not seeing an obvious cause, but let me know if you need help debugging and I'll look more closely.

Clicked too soon

[sam3d]

Okay this could take some work. I'm gonna close this for now and revisit this specifically in perhaps a day or two; in the meantime I'm going to test out another solution that I've been working on that I hope to be able to implement into this.