Add Promise support

If a user wants to load user’s inventory in a function, he/she has to do this:

function sendOffer(steamId, itemIds) {
  this.manager.loadUserInventory(steamId, 730, 2, true (loadInvErr, inv) => {
    // blah blah blah
  }
}  

Eventually, if you have a lot of callbacks, it can grow up to a mess, like in these screenshot:

If this library had a Promise support, you can do it this way (that’s how I imagine it):

function sendOffer(steamId, itemIds) {
  this.manager.loadUserInventory(steamId, 730, 2, true)
    .then(result => {
    // doing something with result
    }
    .catch(err => { 
      // handling errors here
    }
} 

Or (that’s the way I prefer to use it) even use async/await with it (I think it’s awesome, though it’s not officially supported by Node.JS/ECMA standard and I have to use babel to compile it):

async function sendOffer(steamId, itemIds) {
  try {
    const result = await this.manager.loadUserInventory(steamId, 730, 2, true)
    // doing something with the result
  } catch (e) {
    // handling errors
  }
}  

We don’t even need to use custom libraries (like q, bluebird etc.), because Node.JS handles Promises natively now.

My suggestion is: every time a function with callback (e.g. offer.getReceivedItems) is called, check the last argument, and if it’s not defined, then return a Promise wrapper, if it’s defined, just call the callback directly.

[DoctorMcKay]

Since v4 is out of active support, I’m (slowly) starting to migrate my stuff to Node v6 (i.e. dropping support for 4). This will be a major module release when it does happen.

[srg-n]

Any updates on this? Looks like v3 has been on hold for quite some time.