Rename viz/index -> kanban

Allows us to implement more visualisations. Eventually it would be nice
to switch between them, but that can come later.
Refactor Redmine to begin work on a burnup chart

Breaks Kanban for the time being.
Add some hints about the downloading
Remove some dead methods + add a flatten method

The flatten() method flattens out the returns from Redmine, which likes
to return fields as objects containing the entity id in the DB and the
value. Custom fields are then returned as an array of those, which is
pretty frustrating.
Make the data processing part of the visualisation

This moves Redmine.js to be a fairly generic way to make RESTful calls
to Redmine, which makes a lot more sense IMO. Separate out the
visualisation class into initialisation, data loading (which calls
Redmine and the visualisation method, which draws the data). Data
processing currently happens somewhere in between load() and
visualise().
Fix the kanban code for the changes to redmine.js