src: Refactoring common.hpp and minimizing exposed includes

Face class has relatively large rewrite to hide internals using
Implementor pattern.  As of this commit, <boost/asio.hpp> is not
automatically included whenever ndn-cxx/face.hpp is included.  If it is
needed to directly work with io_service, asio.hpp should be specifically
included.

Change-Id: Ie742b851025b4e3da634eb981319df0f42937855
diff --git a/tests/transport/dummy-face.hpp b/tests/transport/dummy-face.hpp
index 58b4e81..735702e 100644
--- a/tests/transport/dummy-face.hpp
+++ b/tests/transport/dummy-face.hpp
@@ -14,6 +14,7 @@
 #define NDN_TESTS_TRANSPORT_DUMMY_FACE_HPP
 
 #include "face.hpp"
+#include "transport/transport.hpp"
 
 namespace ndn {