json generation enhancements

I have not had anyone follow up on this, so I have added docs and will
add this to the commitfest.

Recap:

This adds the following:

json_agg(anyrecord) -> json
to_json(any) -> json
hstore_to_json(hstore) -> json (also used as a cast)
hstore_to_json_loose(hstore) -> json

Also, in json generation, if any non-builtin type has a cast to json,
that function is used instead of the type's output function.

cheers

andrew