Add Unicode guidelines

Based on the discussion in the mailing list thread "Unicode in
python3", this adds a set of Unicode guidelines to the coding
standards.

As a proof of concept, I have updated `astropy.units.Unit` to follow
them (more classes to follow, but I just wanted to get one out the
door for review first).

I used `six` in the example because that's where we're headed, but
didn't use `six` in `astropy.units` because that package has not been
converted over yet.