Cleaning up seek code.

Replacing pts calculation based on duration
with that based on fps for higher precision

add av_register_all() function call back into FFmpegDemuxer.cpp to av…

…oid error about 'no AVFormatContext provided' (NVIDIA#289)

Fixing issues with PyFfmpegDemuxer ctor

Refactoring PyNvCodec module

Merge branch 'on_gpu_buffer'