bytearray cannot pass the Tornado's bytes_type assertion


```python
File "/home/wenhao/workspace/python/WebSocket-for-Python/ws4py/websocket.py", line 175, in send
    self.sender(m)
  File "/usr/local/lib/python2.7/dist-packages/tornado/iostream.py", line 203, in write
    assert isinstance(data, bytes_type)
```

Change the Frame.build to return a string instead.