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.
No need for an OrderedDict here if you use OptionsWidget with sort=True in the form (instead of the SELECT).

Reason: the config.py should not need to provide an OrderedDict (it should not have to deal with Python too much), it is better to have the ordering of the options where it's needed and only there. That's faster and easier for non-coders.

In fact, I think this will become a MultiSelect instead of a simple drop-down so that an alert can be published to multiple channels in one go. And MultiSelect too has a sorting-option.