11 Import Eventbrite Data

Finally finished the importing of the Eventbrite data. 

Running `rake importer:eventbrite:import_event` will give you an overview of all the events on Eventbrite. Enter the number of the event you want to import. This will create the Event, and import all the attendee/tickets. 

Running `rake importer:eventbrite:update_events` will go over each event we currently have and import new attendees/tickets (what it won't do: update existing attendees/tickets).

So for now the way this works: the importer checks whether or not a Ticket exists with the eventbrite ticket id; if it doesn't it will create a Ticket and an Attendee.

What we don't do yet is attaching a new Ticket to an existing Attendee, but we'd need to discuss what the unique identifier of an Attendee actually is. For now the current importer should be enough.

Couple of other changes:
- moved Importers folder to lib
- made some basic changes to Attendee and Ticket model to deal with eventbrite data

Closes #11