Make local presence ID optional

Local presence IDs should be unique. If consumers do not want the
responsibility of taking care of this (and don't care about what ID is
assigned to them), then we will automatically assign a random ID for
them.

Author: Alec Gibson

README.md

@@ -707,7 +707,7 @@ presence.create(presenceId): LocalPresence;
 
 Create an instance of [`LocalPresence`](#class-sharedblocalpresence), which can be used to represent local presence. Many or none such local presences may exist on a `Presence` instance.
 
-* `presenceId` _string_: a unique ID representing the local presence. Remember - depending on use-case - the same client might have multiple presences, so this might not necessarily be a user or client ID.
+* `presenceId` _string (optional)_: a unique ID representing the local presence. Remember - depending on use-case - the same client might have multiple presences, so this might not necessarily be a user or client ID. If one is not provided, a random ID will be assigned for you.
 
 #### `destroy`
 

lib/client/presence/presence.js

@@ -3,6 +3,7 @@ var LocalPresence = require('./local-presence');
 var RemotePresence = require('./remote-presence');
 var util = require('../../util');
 var async = require('async');
+var hat = require('hat');
 
 module.exports = Presence;
 function Presence(connection, channel) {
@@ -35,6 +36,7 @@ Presence.prototype.unsubscribe = function(callback) {
 };
 
 Presence.prototype.create = function(id) {
+  id = id || hat();
   var localPresence = this._createLocalPresence(id);
   this.localPresences[id] = localPresence;
   return localPresence;