WIP: Support Python 3.

This is a work in progress. The packages builds and installs under py3, but it fails on import like so:

    In [1]: import av
    ---------------------------------------------------------------------------
    ImportError                               Traceback (most recent call last)
    <ipython-input-1-55f15964059f> in <module>()
    ----> 1 import av

    /Users/danielallan/Documents/Repos/PyAV/av/__init__.py in <module>()
          8 
          9 # For convenience, IMPORT ALL OF THE THINGS (that are constructable by the user).
    ---> 10 from .audio.fifo import AudioFifo
         11 from .audio.format import AudioFormat
         12 from .audio.frame import AudioFrame

    /Users/danielallan/container.pxd in init av.audio.fifo (/Users/danielallan/Documents/Repos/PyAV/src/av/audio/fifo.c:2743)()

    /Users/danielallan/frame.pxd in init av.container (/Users/danielallan/Documents/Repos/PyAV/src/av/container.c:5922)()

    /Users/danielallan/stream.pxd in init av.frame (/Users/danielallan/Documents/Repos/PyAV/src/av/frame.c:2108)()

    /Users/danielallan/frame.pxd in init av.stream (/Users/danielallan/Documents/Repos/PyAV/src/av/stream.c:4773)()

    /Users/danielallan/plane.pxd in init av.audio.frame (/Users/danielallan/Documents/Repos/PyAV/src/av/audio/frame.c:2564)()

    ImportError: dlopen(/Users/danielallan/Documents/Repos/PyAV/av/plane.so, 2): Symbol not found: _PyString_FromStringAndSize
      Referenced from: /Users/danielallan/Documents/Repos/PyAV/av/plane.so
      Expected in: dynamic lookup