Add insertFirst option in ol.OverlayOptions typedef
Insert or append overlay based on insertFirst option

This is to fix z-index issues with overlays that pass above ol3 default
controls.
Fix overlay z-index issue in popup example
Fix overlay z-index issue in overlay example
Set z-index: auto so that popup is not above controls
Improve doc for stopEvent and insertFirst options
Overlay stopEvent and insertFirst default to true
Merge pull request #1122 from bbinet/fix-zindex-issues

Fix overlay zindex issues