A/V modifications

CHANGES:
    - Everything is now in one public header: toxav.h,
    - Stream is encrypted,
    - Added endianness support though i only tested it with little endians bigs _should_ work too,
    - Now using vpx. Vpx and opus are the only dependencies required for av support;
      FFmpeg is still required to compile test phone but not the lib.

TODO:
    - More testing