Improve the way lernid discovers learning events

Please see the bug comments for additional information.

Ship one or more config files, each with a description of one or more classrooms. The configs are permanent and contain no times and are designed to specify and connection and the location of the channels. They each contain:

 * a classroom name
 * calendar url
 * classroom and chatroom names

We should really also provide support for other IRC networks, so it might also make sense to include:

 * IRC network

(alternatively we could provide URLs before the classroom and chatroom names)

Perhaps, pull the classroom url from the classroom calendar's X-WR-CALDESC: field.

There would be one config file for each classroom (with associated chat channel). As such for other classrooms (e.g. such as different language), an addition config file is required.

If possible, the config file(s) might go in /etc/lernid/lernid.classrooms.d/ . If so, other distributions or users can release packages to add their classroom config files to this config directory, changing the Lernid user's choices without patching any code.

There would be one calendar per classroom but session entries on that calendar would include event names and they could be presented to the user in the schedule tab.

An example of the text added to the Description of an event in the calendar:

Event: Ubuntu Open Week
Slides: http://jonobacon.org/files/presentation.pdf
Instructors: nick1, nick2
Helpers: nick3, nick4

In the Session tab, Lernid will display sessions no more than seven days old and will be scrolled to the current, (or next) event.

Leaving user classroom configurability for later. In the meantime, lernid can be started with the --classroom and --chatroom options.