Added a simple debug observer.

..makes it a bit easier for me to inspect XML
before actually hitting QuestBack with request when
all I really need is to inspect the built XML.
Better error msg for unknown attributes.
Added enum namespace.

Have not gotten this confirmed by QuestBack, but
it seems to work.
Completed support for #add_respondents_data.

I have to make a comment on #transform_hash_for_quest_back.
Yes, it is really hackish, but no I ain't gonna
clean it up now.

Also, using #add_respondents_data requires
a huge hash with correct attributes. A better solution
will be to create ourself som structure (classes)
which we can initiate instead of manually putting
the responsibility on the client of this API to
create exactly correct hashes.

This will have to wait for another time :-(