Improved userfeedback during chunking phase of the file adding process.

This patch provides additional events for observing chunking 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 while javascript programms run. This is rather 'user unfriendly when the program runs for a long time' (like when you create all the chunk objects for a large upload). This problem can be fixed by running parts of the long-running code via window.setTimeout so the second part of the patch does exactly this.