Fixes #15.

* spot/twaalgos/minimize.hh, spot/twaalgos/minimize.cc
(minimize_obligation_garanteed_to_work): New function.
* spot/twaalgos/postproc.hh, spot/twaalgos/postproc.cc: Use it if
wdba-minimize=1.  Handle new default for wdba-minimize.
* NEWS, bin/spot-x.cc: Document those changes.
* tests/core/ltl2tgba2.test: Add some test cases.
* tests/core/genltl.test: Improve expected results.