User visible changes
--------------------

Add requirements for topic handling.

Add topic handling.  The output document is now handles completly
different: each requirement points to a topic where it belongs
to.  (Note that this is not compatible with the old versions of rmtoo
input files any more.)


Internal Changes
----------------

Refactored the Parser class: it now supports parsing of files with
tags which appear many times.  This is needed for the topic feature.

Add more unit-tests.
