subscribe(event, callback) {
        
        let self = this;
        
        // If there's not already an event with this name set in our collection
        // go ahead and create a new one and set it with an empty array, so we don't
        // have to type check it later down-the-line
        if(!self.events.hasOwnProperty(event)) {
            self.events[event] = [];
        }
        
        // We know we've got an array for this event, so push our callback in there with no fuss
        return self.events[event].push(callback);
    }
Either create a new event instance for passed `event` name
or push a new callback into the existing collection

@param {string} event
@param {function} callback
@returns {number} A count of callbacks for this event
@memberof PubSub