rename src/ as spot/ and use include <spot/...>

* NEWS: Mention the change.
* src/: Rename as ...
* spot/: ... this, adjust all headers to include <spot/...> instead of
"...", and adjust all Makefile.am to search headers from the top-level
directory.
* HACKING: Add conventions about #include.
* spot/sanity/style.test: Add a few more grep to catch cases
that do not follow these conventions.
* .gitignore, Makefile.am, README, bench/stutter/Makefile.am,
bench/stutter/stutter_invariance_formulas.cc,
bench/stutter/stutter_invariance_randomgraph.cc, configure.ac,
debian/rules, doc/Doxyfile.in, doc/Makefile.am,
doc/org/.dir-locals.el.in, doc/org/g++wrap.in, doc/org/init.el.in,
doc/org/tut01.org, doc/org/tut02.org, doc/org/tut03.org,
doc/org/tut10.org, doc/org/tut20.org, doc/org/tut21.org,
doc/org/tut22.org, doc/org/tut30.org, iface/ltsmin/Makefile.am,
iface/ltsmin/kripke.test, iface/ltsmin/ltsmin.cc,
iface/ltsmin/ltsmin.hh, iface/ltsmin/modelcheck.cc,
wrap/python/Makefile.am, wrap/python/ajax/spotcgi.in,
wrap/python/spot_impl.i, wrap/python/tests/ltl2tgba.py,
wrap/python/tests/randgen.py, wrap/python/tests/run.in: Adjust.

README

@@ -122,7 +122,7 @@ Some documentation can be found in the doc/ directory.
   logic operators supported by Spot
 
 "make install" will install man pages for command-line tools.  (These
-man pages can also be found in the src/bin/man/ subdirectory of the
+man pages can also be found in the spot/bin/man/ subdirectory of the
 source tree.)  Additional documentation about these tools can be found
 in doc/userdoc/, or on-line at http://spot.lrde.lip6.fr/tools.html
 
@@ -134,7 +134,7 @@ Layout of the source tree
 Core directories
 ----------------
 
-src/              Sources for libspot.
+spot/             Sources for libspot.
    bin/           User tools built using the Spot library.
       man/        Man pages for the above tools.
    graph/         Graph representations.