example: Add a repeating timer sample

Tested:
    Ran through unit test suite and manually executed the example
    program to make sure it works as expected.

Change-Id: I77cffdd038df4eab774f0d162f49273650638ad6
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
index b91a4df..d9423e4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -43,6 +43,7 @@
 # Output binaries
 /example/follow
 /example/heartbeat
+/example/heartbeat_timer
 /test/clock
 /test/event
 /test/exception