Allow tagged artefacts to be grouped by format type

Add a new presenter, GroupedResultSetPresenter, which will group the 
items in the provided result set by their kind/format. 

Requests to /with_tag.json?group_by=format will receive a different 
response built by this presenter, instead of the regular 
ResultSetPresenter.

For now, the list of groupings is hard-coded in the new presenter, along 
with the labels which should be returned for each group.