Use list of words in transfer method

The methods and documentation were converted from using strings to using lists of words in a previous commit, but unfortunately I neglected to completely convert the transfer method. This commit makes the transfer method match that behavior as well.

Fixes #3

Author: tomstokes

spi.py

@@ -421,8 +421,9 @@ def transfer(self, data, speed=0, bits_per_word=0, delay=0):
         Returns:
             List of words read from SPI bus during transfer
         """
+        data = array.array('B', data).tostring()
         length = len(data)
-        transmit_buffer = ctypes.create_string_buffer(str(data))
+        transmit_buffer = ctypes.create_string_buffer(data)
         receive_buffer = ctypes.create_string_buffer(length)
         spi_ioc_transfer = struct.pack(SPI._IOC_TRANSFER_FORMAT,
                                        ctypes.addressof(transmit_buffer),