Expose the map renderer type to the public API

In the case  where the map decides itself which renderer to use. It can
be convenient for the developer to have knowledge of the choice made.
Rename ol.RendererHint to ol.RendererType
Provide ol.RendererType structure through ol.renderer.Map

It was previously in ol.Map, but it could not be provided because
of circular dependencies.
Rename Map.DEFAULT_RENDER_HINTS to Map.DEFAULT_RENDERER_TYPES
Export ol.ObjectEvent#key property
Update year in LICENSE
Add "img" option to ol.style.Icon
Merge pull request #2150 from probins/patch-1

Update year in LICENSE
Merge pull request #2112 from elemoine/iconload

Icon image loading related exports
Add getTileUrlFunction in ol.source.TileImage
Add getTileLoadFunction and setTileLoadFunction in ol.source.TileImage
Merge pull request #2133 from fredj/ol.ObjectEvent.key

Export ol.ObjectEvent#key property
Document Map.render() and Map.renderSync()
Add ol.FeatureOverlay tests
Merge pull request #2155 from probins/rendersync

Document Map.render() and Map.renderSync()
Merge pull request #2121 from oterral/utils_func_cache

Add getters and setters for TileUrlFunction and TileLoadFunction properties
Merge pull request #2156 from elemoine/featureoverlaytests

Add ol.FeatureOverlay tests
Enhance and improve the accessibility example.

Changes to improve the accessibility example:

 - remove the `accesskey` attributes. Use of the `accesskey` is generally not recommended as it tends to cause conflicts with AT software and user settings.
 - Change the scripted "Go to map" accesskey to a regular skiplink.
 - Use script to inject the buttons below the map, if the map were to fail for lack of javascript support the buttons should not be there either. These should be buttons (and not hyperlinks) because they provide an action not a navigation.
 - Update the docs.
 - Add an empty `alt` attribute to the logo since it is strictly decorative (this should probably be done sitewide - which IMO is out of scope for this PR)
Make getFeatureById method exportable
Add interface for map events so properties can be disambiguated