setup.py: explicitly read README.rst as UTF-8

For whatever reason, Python 3 doesn't assume UTF-8 decoding in Mac OS.
README.rst is encoded as UTF-8, so use this encoding regardless of the
system locale/default encoding.

The builtin `open` function doesn't support an `encoding` argument in
Python 2, though, so swap it out for `codecs.open`.
Merge pull request #153 from hfalcic/master

setup.py: explicitly read README.rst as UTF-8