Add category support.

This adds:

* CategoryChannel, which represents a category
* Guild.by_category() which traverses the channels grouping by category
* Guild.categories to get a list of categories
* abc.GuildChannel.category to get the category a channel belongs to
* sync_permissions keyword argument to abc.GuildChannel.edit to sync
  permissions with a pre-existing or new category
* category keyword argument to abc.GuildChannel.edit to move a channel
  to a category

discord/state.py

@@ -484,6 +484,10 @@ class ConnectionState:
 
     def parse_channel_create(self, data):
         factory, ch_type = _channel_factory(data['type'])
+        if factory is None:
+            log.warning('CHANNEL_CREATE referencing an unknown channel type %s. Discarding.', data['type'])
+            return
+
         channel = None
 
         if ch_type in (ChannelType.group, ChannelType.private):
@@ -496,10 +500,6 @@ class ConnectionState:
             guild_id = utils._get_as_snowflake(data, 'guild_id')
             guild = self._get_guild(guild_id)
             if guild is not None:
-                if factory is None:
-                    log.warning('CHANNEL_CREATE referencing an unknown channel type %s. Discarding.', data['type'])
-                    return
-
                 channel = factory(guild=guild, state=self, data=data)
                 guild._add_channel(channel)
                 self.dispatch('guild_channel_create', channel)