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/guild.py

@@ -250,11 +250,11 @@ class Guild(Hashable):
             channels = data['channels']
             for c in channels:
                 if c['type'] == ChannelType.text.value:
-                    channel = TextChannel(guild=self, data=c, state=self._state)
-                    self._add_channel(channel)
+                    self._add_channel(TextChannel(guild=self, data=c, state=self._state))
                 elif c['type'] == ChannelType.voice.value:
-                    channel = VoiceChannel(guild=self, data=c, state=self._state)
-                    self._add_channel(channel)
+                    self._add_channel(VoiceChannel(guild=self, data=c, state=self._state))
+                elif c['type'] == ChannelType.category.value:
+                    self._add_channel(CategoryChannel(guild=self, data=c, state=self._state))
 
 
     @property
@@ -309,6 +309,52 @@ class Guild(Hashable):
         r.sort(key=lambda c: c.position)
         return r
 
+    @property
+    def categories(self):
+        """List[:class:`CategoryChannel`]: A list of categories that belongs to this guild.
+
+        This is sorted by the position and are in UI order from top to bottom.
+        """
+        r = [ch for ch in self._channels.values() if isinstance(ch, CategoryChannel)]
+        r.sort(key=lambda c: c.position)
+        return r
+
+    def by_category(self):
+        """Returns every :class:`CategoryChannel` and their associated channels.
+
+        These channels and categories are sorted in the official Discord UI order.
+
+        If the channels do not have a category, then the first element of the tuple is
+        ``None``.
+
+        Returns
+        --------
+        List[Tuple[Optional[:class:`CategoryChannel`], List[:class:`abc.GuildChannel`]]]:
+            The categories and their associated channels.
+        """
+        grouped = {}
+        for channel in self._channels.values():
+            if isinstance(channel, CategoryChannel):
+                continue
+
+            try:
+                channels = grouped[channel.category_id]
+            except KeyError:
+                channels = grouped[channel.category_id] = []
+
+            channels.append(channel)
+
+        def key(t):
+            k, v = t
+            return (k.position if k else -1, v)
+
+        _get = self._channels.get
+        as_list = [(_get(k), v) for k, v in grouped.items()]
+        as_list.sort(key=key)
+        for _, channels in as_list:
+            channels.sort(key=lambda c: c.position)
+        return as_list
+
     def get_channel(self, channel_id):
         """Returns a :class:`abc.GuildChannel` with the given ID. If not found, returns None."""
         return self._channels.get(channel_id)