graph: Implement a directed graph.

* src/graph/graph.hh, src/graph/Makefile.am, src/graphtest/graph.cc,
src/graphtest/graph.test, src/graphtest/defs.in,
src/graphtest/Makefile.am: New files.
* src/Makefile.am, configure.ac: Add graph/ and graphtest/.
* README: Mention these directories.

src/graph/Makefile.am

@@ -0,0 +1,27 @@
+## -*- coding: utf-8 -*-
+## Copyright (C) 2014 Laboratoire de Recherche et Développement de
+## l'Epita (LRDE).
+##
+## This file is part of Spot, a model checking library.
+##
+## Spot is free software; you can redistribute it and/or modify it
+## under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 3 of the License, or
+## (at your option) any later version.
+##
+## Spot is distributed in the hope that it will be useful, but WITHOUT
+## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+## or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+## License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+AM_CPPFLAGS = -I$(srcdir)/..
+AM_CXXFLAGS = $(WARNING_CXXFLAGS)
+
+graphdir = $(pkgincludedir)/graph
+
+graph_HEADERS = \
+  graph.hh