Fix inheriting of static members.

When no explicit staticality stated, try to first inherit from instance
members.  Only when that fails look for static members too.
Ensure tag/* classes are loaded just once.

Instantiate the TagRegistry in Options class to prevent instantiation of
it when we're running the code in parallel forks.
Fix double-loading of Tag::Tag class in Ruby 1.8.

Ruby 1.8 doesn't understand that we have loaded this class already
and will re-load it and then give warnings.