Allow call to Pubnub.publish() without a callback

Currently, the _request method assumes a callable callback parameter.  However, the Pubnub API (and general pub/sub semantics) allows calls to publish() without a callback (see e.g. common/PubnubBase.py:154-158).  Thus _request can be invoked with callback=None.  This raises a "TypeError: 'NoneType' object is not callable" exception at line 70

The proposed change will only invoke the callback when callback != None