Add ol.View2D#getZoom
Export ol.View2D#getZoom
Merge pull request #858 from elemoine/getzoom

Add ol.View2D#getZoom
Replace the a tag inside a div with a button
Add .ol-button-group CSS class name
Add ol.control.Zoom#zoomByDelta function

To replace ol.control.Zoom#handleIn_ and ol.control.Zoom#handleOut_.