Added ability to send notifications to core.py

This will send a notification to the URL that notification is set equal to in the create function.  Then a notification from Zencoder will be sent after the encoding is complete.  If the request looks like this:

client = Zencoder('API_KEY')
        # configure outputs with dictionaries
        mp4 = {
                     'label': ['converted_to_mp4_video', video_id],
                     'format': 'mp4'
                 }
        webm = {
                  'label': ['converted_to_webm_video', video_id],
                  'format': 'webm'
              }
        notifications = "http://www.MY_URL.com/videonotification"
        
        # the outputs kwarg requires an iterable
        outputs = (mp4, webm)
        response_from_job = client.job.create('http://www.THE-URL-WHERE-MY-VIDEO-IS.com'), 
                                              outputs=outputs, 
                                              notifications=notifications)

Then the notification will look like this:

{u'input': {u'height': 240, u'frame_rate': 15.140000000000001, u'format': u'mpeg4', u'audio_bitrate_in_kbps': 12, u'md5_checksum': None, u'video_bitrate_in_kbps': 390, u'channels': u'1', u'width': 320, u'state': u'finished', u'audio_codec': u'amrnb', u'id': 53734579, u'video_codec': u'mpeg4', u'file_size_in_bytes': 806745, u'duration_in_ms': 15850, u'total_bitrate_in_kbps': 402, u'audio_sample_rate': 8000}, u'job': {u'pass_through': None, u'created_at': u'2013-07-26T20:19:07Z', u'updated_at': u'2013-07-26T20:19:41Z', u'state': u'finished', u'test': True, u'id': 53756596, u'submitted_at': u'2013-07-26T20:19:07Z'}, u'outputs': [{u'height': 240, u'frame_rate': 16.670000000000002, u'format': u'mpeg4', u'url': u'https://zencoder-temp-storage-us-east-1.s3.amazonaws.com/o/20130726/d933178672e1a38a4f6e3e0110264a5a/593357c377620f9cc8dec7b4ef998282.mp4?AWSAccessKeyId=AKIAI456JQ76GBU7FECA&Signature=Xwtrh0dIwcrC5wISIsen0XwmEVg%3D&Expires=1374956381', u'audio_bitrate_in_kbps': 10, u'label': [u'mp4', u'5859056946249728'], u'md5_checksum': None, u'video_bitrate_in_kbps': 564, u'channels': u'1', u'width': 320, u'state': u'finished', u'audio_codec': u'aac', u'id': 116650623, u'video_codec': u'h264', u'file_size_in_bytes': 368736, u'duration_in_ms': 5000, u'total_bitrate_in_kbps': 574, u'audio_sample_rate': 8000}, {u'height': 240, u'frame_rate': 16.670000000000002, u'format': u'webm', u'url': u'https://zencoder-temp-storage-us-east-1.s3.amazonaws.com/o/20130726/ccad2000c1519db4c23614926aa1731c/29a90d96a39fe3b24ea0c929c06cc951.webm?AWSAccessKeyId=AKIAI456JQ76GBU7FECA&Signature=Y1IzmHGO7rHQRMijmxuOVPckKm4%3D&Expires=1374956367', u'audio_bitrate_in_kbps': 22, u'label': [u'webm', u'5859056946249728'], u'md5_checksum': None, u'video_bitrate_in_kbps': 490, u'channels': u'1', u'width': 320, u'state': u'finished', u'audio_codec': u'vorbis', u'id': 116650624, u'video_codec': u'vp8', u'file_size_in_bytes': 341938, u'duration_in_ms': 5000, u'total_bitrate_in_kbps': 512, u'audio_sample_rate': 8000}]}

Which gives you a great deal of information about the video and my project couldn't function without the notification feature.