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 
You're probably right, I must have pasted twice - but the syntax is valid :). Will fix, and lower the total size to 1024.