Add support for audit log reasons.

Most routes now have a 'reason' keyword argument.

discord/message.py

@@ -407,7 +407,7 @@ class Message:
                 return '{0.author.name} started a call \N{EM DASH} Join the call.'.format(self)
 
     @asyncio.coroutine
-    def delete(self):
+    def delete(self, *, reason=None):
         """|coro|
 
         Deletes the message.
@@ -416,6 +416,11 @@ class Message:
         delete other people's messages, you need the :attr:`Permissions.manage_messages`
         permission.
 
+        Parameters
+        ------------
+        reason: Optional[str]
+            The reason for deleting this message. Shows up on the audit log.
+
         Raises
         ------
         Forbidden
@@ -423,7 +428,7 @@ class Message:
         HTTPException
             Deleting the message failed.
         """
-        yield from self._state.http.delete_message(self.channel.id, self.id)
+        yield from self._state.http.delete_message(self.channel.id, self.id, reason=reason)
 
     @asyncio.coroutine
     def edit(self, **fields):