Add ToString for time_points

Adds ToString helper functions for aos's time_point classes that match
the format that LogReader expects for start/end_time.

Change-Id: Ied8635da70ceabb82328ea47b34f878084663bdd
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/time/time.h b/aos/time/time.h
index 1a5cbd1..8462625 100644
--- a/aos/time/time.h
+++ b/aos/time/time.h
@@ -81,6 +81,9 @@
 std::ostream &operator<<(std::ostream &stream,
                          const aos::realtime_clock::time_point &now);
 
+std::string ToString(const aos::monotonic_clock::time_point &now);
+std::string ToString(const aos::realtime_clock::time_point &now);
+
 namespace time {
 #ifdef __linux__