AU: Tarjan's algorithm.

Add an implementation of Tarjan's algorith for finding Strongly
Connected Components in a graph. This version is slightly modified: it
takes in a vertex in addition to the graph and returns only the SCC
containing that vertex.

Review URL: http://codereview.chromium.org/657055
diff --git a/SConstruct b/SConstruct
index e92a895..5bfbec7 100644
--- a/SConstruct
+++ b/SConstruct
@@ -103,6 +103,7 @@
                    postinstall_runner_action.cc
                    set_bootable_flag_action.cc
                    subprocess.cc
+                   tarjan.cc
                    update_check_action.cc
 		               update_metadata.pb.cc
 		               utils.cc""")
@@ -130,6 +131,7 @@
                             postinstall_runner_action_unittest.cc
                             set_bootable_flag_action_unittest.cc
                             subprocess_unittest.cc
+                            tarjan_unittest.cc
                             test_utils.cc
                             update_check_action_unittest.cc
                             utils_unittest.cc""")