Add common sd_event timer class

timer.hpp and timer.cpp are used throughout the project by copying them
into whatever git repo they are needed. This puts a header-only
implementation into sdbusplus, which is also used by every project.
Then, if you want a timer, you include sdbusplus/timer.hpp and you are
good to go.

Change-Id: Ica7543ecb66128b645f609e790fa7183eeb34ac1
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/Makefile.am b/Makefile.am
index 32fe3ec..ab07920 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -24,6 +24,7 @@
 	sdbusplus/server/manager.hpp \
 	sdbusplus/server/object.hpp \
 	sdbusplus/slot.hpp \
+	sdbusplus/timer.hpp \
 	sdbusplus/utility/container_traits.hpp \
 	sdbusplus/utility/read_into_tuple.hpp \
 	sdbusplus/utility/tuple_to_array.hpp \