Enforce user defined maximum payload limit for issue #28

- Limit can be set as WebSocketCommonProtocol.max_msglen
  attribute
- read_frame()/read_data_frame()/read_message() now
  accept an optional parameter maxsize
- New exception PayloadTooLarge

for issue #28 
In the grand scheme of things 64kB isn't a lot of RAM but the test would be faster and just as valid with 1024 times less data.