properly decode README.rst and HISTORY.rst

HISTORY.rst has non-ASCII characters in it. The open('HISTORY.rst').read() trick works fine on Python 2, where you get the literal UTF-8 bytes. On Python 3, though, you at the mercy of the default file encoding, which may be ASCII (if LANG='en_US.ASCII' for example). Therefore, we need to explicitly decode with UTF-8 to prevent error on those poor systems.