Configurable event propagation for overlays and controls

For more context, see: https://groups.google.com/forum/#!topic/ol3-dev/UQeyo6kk-Pk

With this PR, there are now two overlay container divs: one will stop event propagation and the other won't.
Controls and overlays are added to one of these overlay container depending on whether their ``stopEvent`` option is true or false.