Re-implement voice sending.

This is a complete redesign of the old voice code.

A list of major changes is as follows:

* The voice websocket will now automatically reconnect with
  exponential back-off just like the regular Client does.
* Removal of the stream player concept.
* Audio now gracefully pauses and resumes when a disconnect is found.
* Introduce a discord.AudioSource concept to abstract streams
* Flatten previous stream player functionality with the
  VoiceClient, e.g. player.stop() is now voice_client.stop()
* With the above re-coupling this means you no longer have to
  store players anywhere.
* The after function now requires a single parameter, the error,
  if any existed. This will typically be None.

A lot of this design is experimental.

discord/channel.py

@@ -290,7 +290,7 @@ class TextChannel(discord.abc.Messageable, discord.abc.GuildChannel, Hashable):
                     count += 1
                     ret.append(msg)
 
-class VoiceChannel(discord.abc.GuildChannel, Hashable):
+class VoiceChannel(discord.abc.Callable, discord.abc.GuildChannel, Hashable):
     """Represents a Discord guild voice channel.
 
     Supported Operations:
@@ -335,6 +335,12 @@ class VoiceChannel(discord.abc.GuildChannel, Hashable):
     def __repr__(self):
         return '<VoiceChannel id={0.id} name={0.name!r} position={0.position}>'.format(self)
 
+    def _get_voice_client_key(self):
+        return self.guild.id, 'guild_id'
+
+    def _get_voice_state_pair(self):
+        return self.guild.id, self.id
+
     def _update(self, guild, data):
         self.guild = guild
         self.name = data['name']