Clarify compiler exports in tasks readme
Export method to get resolution for extent and size

While this is straightforward to calculate, this method adds convenience.
Merge pull request #2141 from probins/patch-5

Clarify compiler exports in tasks readme.
Merge pull request #2142 from tschaub/res-for-extent

Export method to get resolution for extent and size.
Apidoc readme: clarify exportable methods
Merge pull request #2140 from probins/patch-3

Apidoc readme: clarify exportable methods.
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
Add docs link to loadingstrategy
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)