Exponential scaling to ZoomSlider

This PR suggests a number of changes:

* Add getResolutionForValueFunction and getValueForResolutionFunction functions to ol.View2D. The "value for resolution" function returns a value between 0 and 1 for a given resolution. The "resolution for value" function does the opposite. Both assume exponential scaling.
* Make the zoom slider control use these functions. This removes the necessity to configure the control with min and max resolutions.
* Apply the resolution constraint when dragging the slider's thumb, and animate to the final resolution when releasing the thumb.

Some issues will need attention:

* Address "FIXME do we really need this affine transform?" in the zoom slider.
* We probably want to set a `direction` when calling `view.zoom()`.
* We may want to add a `opt_linerar` option or something to getValueForResolutionFunction and getResolutionForValueFunction.

References #396.