add encoding for python 3.x

Installation with python 3.x will fail if command line encoding is not utf-8:

```
$ LANG=C python setup.py build
Traceback (most recent call last):
  File "setup.py", line 15, in <module>
    long_description=open("README.rst").read() + "\n" +
  File "/usr/lib/python3.3/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 1968: ordinal not in range(128)
```

This patch forced encoding to utf-8 if using python 3.x to install. Tried with python 2.7.6 and 3.3.3 and it works well.