add binding for language_containment_checker and document them

* spot/tl/contain.cc, spot/tl/contain.hh: Simplify the
use of language_containment_checker by adding default argument.
* python/spot/__init__.py, python/spot/impl.i: Bind it in Python.
* doc/org/tut04.org: New file to illustrate it.
* doc/org/tut.org, doc/Makefile.am: Add it.
* NEWS: Mention those changes.

NEWS

@@ -38,7 +38,7 @@ New in spot 2.0.1a (not yet released)
     --eh-patterns=1..12, and --sb-patterns=1..27.  Unlike other options
     these do not output scalable patterns, but simply a list of formulas
     appearing in these three papers: Dwyer et al (FMSP'98), Etessami &
-    Holzmann (Concur'00) Somenzi & Bloem (CAV'00).
+    Holzmann (Concur'00), Somenzi & Bloem (CAV'00).
 
   Library:
 
@@ -53,6 +53,9 @@ New in spot 2.0.1a (not yet released)
     is_inherently_weak() will update the corresponding properties of
     the automaton as a side-effect of their check.
 
+  * language_containment_checker now has default values for all
+    parameters of its constructor.
+
   Python:
 
   * The __format__() method for formula support the same
@@ -60,6 +63,13 @@ New in spot 2.0.1a (not yet released)
     So "{:[i]s}".format(f) is the same as
     "{:s}".format(f.unabbreviate("i")).
 
+  * Bindings for language_containment_checker were added.
+
+  Documentation:
+
+  * A new example page shows how to test the equivalence of
+    two LTL/PSL formulas.  https://spot.lrde.epita.fr/tut04.html
+
   Bug fixes:
 
   * Fix compilation error observed with Clang++ 3.7.1 and GCC 6.1.1