Fixes #39, reported by Joachim Klein.

* src/ltlvisit/lunabbrev.hh, src/ltlvisit/lunabbrev.cc: Take an option
to specify which of xor/equiv/implies should be rewritten.
* src/ltlvisit/print.cc (print_spin): Rewrite only xor.
* src/tests/ltlfilt.test: Add a test case.
* NEWS: Mention this.