Merge pull request #1610 from mdboom/unit/cgs_si_properties

CompositeUnit has no value attribute
Write Unicode guidelines.  Add UNICODE_OUTPUT config option.  Add test
helper to test against guidelines.  Modify astropy.units.Unit to follow
those guidelines, and fix a bug discovered when doing that.
Test roundtrip of __repr__ as well.
Add documentation for repr_namespace
Add changelog entry
Do all roundtrip testing or none.
Fixup arguments to assert_follows_unicode_guidelines
Fixups after rebase
Add missing reference target
Fix reference source