expose isArray function

PUBNUB.isArray function would be useful for the code snippet below since this accounts for the Array-like arguments object. I would normally use isArray method on the Array constructor which doesn't account for arguments object edge case
```javascript
PUBNUB._publish = PUBNUB.publish;
PUBNUB.publish = function(body) {
  var channels = body.channel;
  if (typeof channels === 'string' && (/,/i).test(channels)) {
    this.publish(channels.split(','));
  } else if (this.isArray(channels)) {
    this.each(Array.prototype.slice.call(channels, 0), function(channel) {
      body.channel = channel;
      this.publish(body);
    });
  } else {
    this._publish(body);
  }
};
```