Copied the treemap from openspendingjs and added a new context property called drilldown (and can be overwritten in context).

This property is a function and gets called when a node is clicked. Default behaviour is the same behaviour as in openspendingjs' treemap.
Call the new treemap from oak11-13test
Removed redundant initialisations of the treemap for the dataset
Moved the javascript code to main javascript file (could be moved into specific file). Updated oak11-13test accordingly.
Modified the javascript to reload the site with url parameters that define where we've drilled until now. This creates a unique url for each drilldown.

Can be improved (e.g. the alert when no children are present or let user know where he/she is at without having to 'parse' it from the url parameters.

This introduces a dependency on purl.js (jQuery based url parser). It could be implemented easily but why go through the extra work?