For large files, the time spent in creating chunk objects is pretty substatial. This patch provides additional
events for observing this process:

+* `.chunkingStart(file)` Started preparing file for upload
+* `.chunkingProgress(file,ratio)` Show progress in file preparation
+* `.chunkingComplete(file) File is ready for upload

The browser interface does not get updated during long running javascript programms (like the one which
create all the chunk objects). This can be fixed by running parts of the long-running code via window.setTimeout
so the second part of the patch does exactly this.