Fix setup issue on windows with py3.3 where wrong file encoding was used

This came up when trying to install the pypi package on windows with python 3.3.

     Traceback (most recent call last):
       File "<string>", line 17, in <module>
       File "C:\...\env\build\humanize\setup.py", line 18, in <module>
         long_description=open('README.rst').read(),
       File "C:\Python33\lib\encodings\cp1252.py", line 23, in decode
         return codecs.charmap_decode(input,self.errors,decoding_table)[0]
     UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 2017: character maps to  <undefined>
     Complete output from command python setup.py egg_info:
     Traceback (most recent call last):
    File "<string>", line 17, in <module>
    File "C:\...\env\build\humanize\setup.py", line 18, in <module>
      long_description=open('README.rst').read(),
    File "C:\Python33\lib\encodings\cp1252.py", line 23, in decode
      return codecs.charmap_decode(input,self.errors,decoding_table)[0]
    UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 2017: character maps to <undefined>