fix(setup): fix encoding error during install

Encoding error occurs during install from sources on OS X Maverick, python 3.4.2
Error Traceback :
python setup.py install
Traceback (most recent call last):
  File "setup.py", line 89, in <module>
    with open(readme) as f : long_description = f.read()
  File "/usr/local/Cellar/python3/3.4.2_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xef in position 0: ordinal not in range(128)

Fixed this by replacing open(readme) by io.open(readme, encoding='utf-8')