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/enums.py

@@ -31,10 +31,11 @@ __all__ = ['ChannelType', 'MessageType', 'VoiceRegion', 'VerificationLevel',
            'AuditLogAction', 'AuditLogActionCategory', 'UserFlags', ]
 
 class ChannelType(Enum):
-    text    = 0
-    private = 1
-    voice   = 2
-    group   = 3
+    text     = 0
+    private  = 1
+    voice    = 2
+    group    = 3
+    category = 4
 
     def __str__(self):
         return self.name