Add ol.TileLoadFunctionType
Allow tile load function to be overridden
Add tileLoadFunction option to ol.source.XYZ
Export ol.Tile#getTileCoord
Add ol.TileCoord#getZXY
Export ol.TileCoord#getZXY
Export ol.ImageTile#getImage
Merge pull request #1036 from twpayne/async-tile-load

Asynchronos tile URL calculation