Begin working on gateway v6 changes.

The first batch of changes are related to channel types and group
direct messages. Support these first so READY begins parsing.

discord/enums.py

@@ -27,11 +27,13 @@ DEALINGS IN THE SOFTWARE.
 from enum import Enum
 
 class ChannelType(Enum):
-    text = 'text'
-    voice = 'voice'
+    text    = 0
+    private = 1
+    voice   = 2
+    group   = 3
 
     def __str__(self):
-        return self.value
+        return self.name
 
 class ServerRegion(Enum):
     us_west = 'us-west'