OpusStatistics implemented

* Implemented OpusStatistics.
* Changed OpusInfoTool to used the OpusStatistics.
* Adapted the tests.

I needed access to the OggPacket in the inner logic,as well as  the initial info from OpusFile, so I introduced some package level accessors for the needed data. in addition I did a cast from OggStreamAudioData to OpusAudioData. I don't know if this is the way it is supposed to work, please review this and let me know what you think.

I also left the base OggAudioStatistics untouched in OpusStatistics, but they seem to be a bit off.