Introduce a simple timer API....

One can use add_timer or schedule_timer to add a timer. After
the timeout time has been reached the callback will be called.
One can call add_time/schedule_timer and del_timer from within
the callback.
diff --git a/tests/Makefile.am b/tests/Makefile.am
new file mode 100644
index 0000000..f7cfd54
--- /dev/null
+++ b/tests/Makefile.am
@@ -0,0 +1 @@
+SUBDIRS = timer