Added Mutex.lock(key, timeout, cb) function

[zlumer]

Solves #11
Usage:

var lock = require('locks').Mutex.lock;
lock('user_123', function(done) {
  function doSomethingAsync() {
    // ... do something async
    done();
  }
  setTimeout(doSomethingAsync, 500);
});

Custom timeout:

var lock = require('locks').Mutex.lock;
lock('user_123', 20000, function(done) {
  function doSomethingAsync() {
    // ... do something async
    done();
  }
  setTimeout(doSomethingAsync, 10000);
});

[zlumer]

Inspired by redis-lock module.
Based on the code by @tjenkinson.

[gitawego]

I think it's a quite useful addition, please consider to merge it.

[ronkorving]

I see the value in this. But I don't think this belongs inside this repo. Some people may want to lock on objects, not strings, (as you can do in C# if I'm not mistaken). I'm also not a fan of the default timeout hard-coded in this project. To me, those are userland decisions.

I think that increasing the scope of this project should be limited to adding functionality that can't be easily created around it. In this case, it can be. Also, please keep in mind that Mutex is just one of the various synchronization, so if we would go down this path, I would like it to be a more fundamental improvement to the entire library.

Sorry to disappoint, and while I appreciate the engagement, for the sake of protecting the scope of this project, I'm going to close this.