This moves pssh parsing to its own class in media base. This type
can be used to parse and generate pssh boxes. This new type also
supports both v0 an v1 boxes.
This also modifies the pssh box definition to only have the |raw_box|
and moves all pssh parsing and handling into mp4 media parser.
A follow-up change will replace the remaining usages of 'raw' pssh
box data with the new type.
Issue #88
Change-Id: Ic2436ecb5df8b3558b81e600dc095b0efd122ab1
Jacob Trimble