dhcp client: event dispatcher

Add the event dispatcher abstract class and a subclass wrapping
the message loop implementation.

Bug: 25642025
TEST=compile
Change-Id: Ic1e0bbac3e4e1cbe376c8c8a507f2b95b27f57c3
diff --git a/dhcp_client.gyp b/dhcp_client.gyp
index 0346750..f17e85b 100644
--- a/dhcp_client.gyp
+++ b/dhcp_client.gyp
@@ -55,6 +55,7 @@
       },
       'sources': [
         'daemon.cc',
+        'message_loop_event_dispatcher.cc',
         'manager.cc',
       ],
     },