Document backwards incompatibilities

Ripping this from #268 (closed) before closing it.

• Document everything backwards incompatible in changelog:
  • Stream.encode returns a list of packets, instead of a single packet.
  • AudioFifo and AudioResampler will raise ValueError if input frames inconsistant pts.
  • CodecContext.rate has been removed, but proxied to VideoCodecContext.framerate and AudioCodecContext.sample_rate. The definition is effectively inverted from the old one (i.e. for 24fps it used to be 1/24 and is now 24/1).
  • Fractions (e.g. time_base, rate) will be None if they are invalid.
  • InputContainer.seek and Stream.seek will raise TypeError if given a float, when previously they converted it from seconds.