Add .gitattributes to force .rst files to LF line endings

[mikewadsten]

setup.py runs a regex which assumes LF line endings over the contents of CHANGELOG.rst. When installing pyopenssl from source (pip install git+...) on Windows, there's a good chance these files will be checked out with CRLF line endings today, which leads to:

AttributeError: 'NoneType' object has no attribute 'group'

We could also address this by having the read_file function in setup.py enforce LF line endings. I think changing codecs.open to just open would do it, since the default behavior of universal newlines would be to translate to LF.