Organisational units, series, events and groups

Pretty massive PR including:

Organisational units:
 * CRUD
 * Getting / Adding / Deleting series from an organisational unit
 * Getting the organisational unit tree

Series:
 * CRUD
 * Adding / Deleting events to/from series

Events:
 * CRUD


It's pretty rought, but I'll go do some self-review (which hopefully gets in before @nicolaasmatthijs gets on the plane)
AFAICT there will be no single URI for displaying an event in timetables, but the RSS spec does demand it. Maybe this should just be a stub page?

(This seems like a minor issue anyway, as it's very unlikely RSS will be exposed.)