refactor tag cloud to use acts_as_taggable_on's tag cloud codepaths.
extract Event::Overview.
narrow interface between controller and view.
move Event.select_for_overview to Event::Overview.
expose hash keys as methods.
extract some private methods.
move event separation into methods.
tweak tests.
refactor event overview tests.
Merge pull request #176 from botandrose/events_overview

Extract Events::Overview