Locators to loaders

[Tevemadar]

Convert URL-providing locators to data-providing loaders. They're async too, so complex, multi-step processing becomes possible

• transformSeries is async too, just in case
• SeriesLoader
• DZILoader
• TileLoader
• AtlasLoader
• AtlasVolumeLoader
• rename locators object
• update inline documentation
• update readme
• update example deployment.