encoding for credits.txt

Hello

building shapely with Python 3.4, i get the following error:

```
Traceback (most recent call last):
  File "setup.py", line 78, in <module>
    credits = fp.read()
  File "/project/avd/python/python3/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 0xc3 in position 870: ordinal not in range(128)
```

I have seen this other places with Python 3.4, where the encoding for a string is not set to utf-8

I have looked in setup.py and it appears to be CREDITS.txt which is the problem

This PR attempts to fix this issue and enable installation for Python 3.4