Use StandardTagParser for @return, @throws, @type.

The parsing now takes a config parameter to turn on/off parsing of Type
and Name sections.

This cleans up the DocScanner class considerably as all those complex
methods are now in StandardTagParser, leaving the DocScanner with
basics such as #look and #match, plus the big #standard_tag.
Make most StandardTagParser methods private.

Let the #hw method return it's original return value, which is the
DocScanner class - then it chained with one of the DocScanner
methods like #look or #match.  The only exception being #default_value
method which belongs to StandardTagParser and can't be therefore
chained from #hw method - but that's just a one little exception
we can live with.