Change all email/password pair examples to use token.

README.md

@@ -57,7 +57,7 @@ async def on_message(message):
         await asyncio.sleep(5)
         await client.send_message(message.channel, 'Done sleeping')
 
-client.run('email', 'password')
+client.run('token')
 ```
 
 Note that in Python 3.4 you use `@asyncio.coroutine` instead of `async def` and `yield from` instead of `await`.