Update AssetManager.as

Hi, 

I have two suggestions for the AssetManager:

1. When loading multiple texture atlases, processing the xml part takes quite some time (specially with extensive xmls). I suggest to process the xmls one by one with a short timeout to give the frame rate a break (as when loading raw assets, cmp. function processNext). 

2. The onProgress callback is invoked as soon as the texture is loaded/added. But xmls are not processed at this point. As mentioned before, this can take quite a bit of time. I added a second parameter to the callback to indicate whether xmls are processed yet. 

What do you think?