fixed memoryview error

Got the following error on OSX ML:

Traceback (most recent call last):
  File "btserver.py", line 17, in <module>
    data = conn.recv(1024)
  File
"/Library/Python/2.7/site-packages/lightblue/_bluetoothsockets.py",
line 470, in recv
    return self.__incomingdata.read(bufsize)
  File
"/Library/Python/2.7/site-packages/lightblue/_bluetoothsockets.py",
line 150, in read
    self._build_str()
  File
"/Library/Python/2.7/site-packages/lightblue/_bluetoothsockets.py",
line 135, in _build_str
    new_string = "".join(self.l_buffer)
TypeError: sequence item 0: expected string, memoryview found
Merge pull request #1 from kappaloris/master

memoryview error on OSX: _bluetoothsockets.py@135