Loader - fast image-from-cache path

Employee the use of image.complete and a width/height check to detect when
it is available "from cache" and skip having to run though the
onload/onerror cases.
Loader - transformUrl

Pulled out baseURL extension into method.
Loader - added enabledParallelDownloads

Added method to enable/disable parallel downloading in general.
Loader - documentation

Updated/corrected various documentation.
Loader - fixes

- `onLoadComplete` will fire, even when loading is interrupted
- Context default to `this` on application.
Loader - added means to add synchronization points

- Added `withSyncPoint` and `addSyncPoint` methods to allow explicit
  adding of synchronization points (synchronization points are explained
  in `withSyncPoint`.

- Changed the file/asset `sync` attribute to `syncPoint` to reflect
  terminology.
Loader - corrected tag error handling