Store the PES before decoding it. Plus changes to decode picture types

From 13818-1, 13818-2 and H264 spec
http://braice.net/sites/default/files/iso13818-1.pdf
http://www.comp.nus.edu.sg/~cs5248/0910S2/l03/IEC-13818-2_Specs.pdf
http://www.itu.int/rec/T-REC-H.264-201304-I/en