Automatic loading of all builtin meta-tags.

MetaTagLoader now only deals with loading and has no other knowledge
of builtin tags than the path ./tag from which it loads when :builtins
is passed to #load method.

The loader is now only accessed through MetaTagRegistry#load and not
directly any more.  This simplifies the tests a little bit - they now
only have to do one simple call to MetaTagRegistry.instance.load.