Image printing error in Python 3, use bytearray()

Thanks for building this. Printing in python2.7 works great.
But in python3 the image printing did not output an image.

It seems the `chr()` function incorrectly encodes integers to bytes in python3.

Replacing the `write_bytes` function with the following works in python3:

```
def write_bytes(self, byte_array):
        msg = bytearray(byte_array)
        self.write(msg)
```