Fix AttributeError: 'ByteBuffer' object has no attribute 'tobytes'

[bdrung]

Following Python code fails:

interface = pyipmi.interfaces.create_interface(interface='ipmitool', interface_type='lanplus')
ipmi = pyipmi.create_connection(interface)
ipmi.session.set_session_type_rmcp(host=fqdn, port=port)
ipmi.session.set_auth_type_user(username=username, password=password)
ipmi.target = pyipmi.Target()
ipmi.session.establish()
ipmi_info = ipmi.get_device_id()

Traceback (most recent call last):
  File "example.py", line 7
    ipmi_info = ipmi.get_device_id()
  File "pyipmi/bmc.py", line 25, in get_device_id
    return DeviceId(self.send_message_with_name('GetDeviceId'))
  File "pyipmi/__init__.py", line 206, in send_message_with_name
    rsp = self.send_message(req)
  File "pyipmi/__init__.py", line 190, in send_message
    rsp = self.interface.send_and_receive(req)
  File "pyipmi/interfaces/ipmitool.py", line 147, in send_and_receive
    py3_array_tobytes(req_data))
  File "pyipmi/utils.py", line 57, in py3_array_tobytes
    return msg.tobytes()
AttributeError: 'ByteBuffer' object has no attribute 'tobytes'

Add a ByteBuffer.tobytes method. ByteBuffer.tobytes will only be called if Python's array.array has a tobytes method. Therefore the helper py3_array_tobytes is not needed.

Fixes #80

[hthiery]

thank you