utility/timer: Implement

We often need a continually ticking timer for our daemons. This utility
wraps an sd_event time source as a convenience. This is meant to be a
usable replacement for the timer.hpp found in other openbmc projects.

Tested:
    New tests pass with full coverage. Changes to the phosphor-watchdog
    that rely on this utility work as expected.

Change-Id: Id12aed9e5b018e7eca825c4a7ac7b4f46e2f04c6
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
index 4d40130..b91a4df 100644
--- a/.gitignore
+++ b/.gitignore
@@ -54,3 +54,4 @@
 /test/source/io
 /test/source/signal
 /test/source/time
+/test/utility/timer