Fix 'grids' type declaration.

grids property is optional.
see: https://github.com/mapbox/tilejson-spec/blob/master/2.0.0/README.md
Merge pull request #235 from fredj/tilejson-externs

Fix 'grids' type declaration.
Remove extent property from ol.tilegrid.TileGridOptions

This follows tilegrid extent_ property removal from #216.
Merge pull request #237 from bbinet/tilegrid_extent_cleanup

Remove extent property from ol.tilegrid.TileGridOptions
Listen to both click and touchend events
Allow touch event (touchstart) to fire click events