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)
We do with `OrgUnit.hasOne(OrgUnit, { 'as': 'Parent', 'foreignKey': 'parentId' });`.

I'll make another pass over all the models and add the foreign key columns explicitly in the `define` call.