Added support for stream limit notification

Hi,
I added support for the "limit" notification type. It gets sent from the stream if the track request matches more tweets than currently allowed to deliver, i.e. 1% of total Twitter volume in case of free "Spritzer" account type. I tested it tracking the keyword "a". The parameter "track" lists the number of tweets missed due to this limit since connecting to the stream.

See: https://dev.twitter.com/streaming/overview/messages-types#limit_notices