Add ol.View2D#getZoom
Zoomslider cannot be added to a map with no target

The zoom slider control's initSlider_ function requires that the control's element is inserted in the document. So if initSlider_ is called before the map have a target then the slider isn't correctly initialized. This commit fixes that by defering the slider initialization until the first handleMapPostrender call.
handleMapPostrender may get a null frameState
Merge pull request #855 from elemoine/zoom-slider-fix

Zoomslider cannot be added to a map with no target
Add ol.control.Zoom#zoomByDelta function

To replace ol.control.Zoom#handleIn_ and ol.control.Zoom#handleOut_ callbacks.
Better docs for View2DOptions
@class doc for View2D
Merge pull request #857 from elemoine/view2d-docs

Better View2D docs
Merge pull request #840 from fredj/zoomcontrol

Add ol.control.Zoom#zoomByDelta function
Export ol.View2D#getZoom
Merge pull request #858 from elemoine/getzoom

Add ol.View2D#getZoom
Getters and setters for line symbolizers
Preserve unexpected items in options object passed to the constructor
Getters and setters for polygon symbolizer
Getters and setters for shape symbolizers
Fixing bug with IconLiteral#equals and adding tests
Getters and setters for icon symbolizer
Adding tests for text symbolizer
Getters and setters for text symbolizer
Merge pull request #861 from fredj/layer-options

Preserve unexpected items in options object passed to the constructor