Upgrade to ffmpeg 2.1.3

Hi,
I've upgraded to ffmpeg 2.1.3, for this:
1. I've changed generate.py script - in this version there is typedef for function pointer (avfilter_execute_func) and it uses in struct's field, so I set IntPtr type for such field
2. Upgrade binaries and headers to ffmpeg 2.1.3
3. Fix version in README.md

Best Regards,
Misha Uliutin