Added publishing functionality to Pubsubhubbub

Added the functionality of sending the alert to Pubsubhubbub.

Right now we have all the valid info only about pubsubhubbub . Will add a broker table later as and when more brokers are available.
Really - why make the channels configurable, but then raise an error if it is not Pubsubhubbub? Either it's configurable and you serve all possible configurations - or it is not configurable and only pubsubhubbub.