Merge pull request #161 from elemoine/nullimage

Do not draw image if image is null
Add equals to Rectangle
Add ol.ImageUrlFunction
Add ol.Image class
Add ol.layer.ImageLayer
Add ol.source.ImageSource
Add ol.renderer.canvas.ImageLayer
Add containsExtent to ol.Extent
Add ol.source.SingleImageWMS
Add wms-singleimage example
Add FIXME in ol.control.Attribution