New OverviewMap control and example, incomplete

Features a working overview map control in which a ol.Map object is
created and bound to the control map (main map) to fit its extent when
the main map resolution and center change.

The overview map doesn't contain a rectangle representing the current
main map extent, it's navigable where it ideally shouldn't (you would
move the rectangle around instead). It doesn't have a
'collapse/expand' button yet.