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