Incoming Event Data Feeds

Using one or more external sources of data to populate the event listings for a given group.

Use Cases:
1) In Cambridge, societies can list their talks on a service called talks.cam. Talks.cam happily gives out its data, but doesn't accept it from elsewhere. Thus, in order for a society to get full exposure for a talk [MyPidge and talks.cam] they'd have to enter the data twice.

Instead, if they are adding a talk, we can redirect them to talks.cam, and then ask them for the URL of their group's list on talks.cam, and pull that data in every 6 hours or whatnot. For any other type of event, we ask them to add it to us.

2) Porterhouse JCR has its own proprietory system for managing event bookings for its events. The JCR publicity officer doesn't want to define an event twice. Their keen webmaster offers to give us an XML feed in a particular format. We can use that to pull in data.

Features:
- Should attempt to detect if the event has already been listed on the Pidge installation: maybe if there is an event the same day that would be enough.
- Should update reasonably regularly, and on specific request via an "Update From Feed" button.