Fixes #187.

* spot/tl/ltlf.cc, spot/tl/ltlf.hh: New files.
* spot/tl/Makefile.am: Add them.
* bin/ltlfilt.cc: Add a new option.
* bin/man/ltlfilt.x: Add bibliographic reference.
* tests/core/ltlfilt.test: Add more tests.
* tests/python/ltlf.py: New file.
* tests/Makefile.am: Add it.
* python/spot/impl.i: Python bindings.
* NEWS: Mention it.