Add package for tree

Add a package for [tree](http://mama.indstate.edu/users/ice/tree/).

It has a Makefile that hardcodes a prefix and some CFLAGS.  Used
filter_file to:

- set the make variable *prefix* to `prefix`; and
- comment out their CFLAGS, just use ours....

It installs, runs on CentOS7, and uninstalls cleanly.

var/spack/repos/builtin/packages/tree/package.py

@@ -0,0 +1,46 @@
+##############################################################################
+# Copyright (c) 2013-2016, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program 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 terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class Tree(Package):
+    """Tree is a recursive directory listing command that produces a depth
+       indented listing of files, which is colorized ala dircolors if
+       the LS_COLORS environment variable is set and output is to
+       tty. Tree has been ported and reported to work under the
+       following operating systems: Linux, FreeBSD, OS X, Solaris,
+       HP/UX, Cygwin, HP Nonstop and OS/2."""
+
+    homepage = "http://mama.indstate.edu/users/ice/tree/"
+    url      = "http://mama.indstate.edu/users/ice/tree/src/tree-1.7.0.tgz"
+
+    version('1.7.0', 'abe3e03e469c542d8e157cdd93f4d8a6')
+
+    def install(self, spec, prefix):
+        filter_file(r'^prefix =.*', 'prefix = %s' % prefix, 'Makefile')
+        filter_file(r'^CFLAGS', '# use spack settings instead... CFLAGS',
+                    'Makefile')
+        make()
+        make('install')