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/src/management/nfd-face-event-notification.hpp b/src/management/nfd-face-event-notification.hpp
index 307c0d9..6432d30 100644
--- a/src/management/nfd-face-event-notification.hpp
+++ b/src/management/nfd-face-event-notification.hpp
@@ -18,6 +18,7 @@
 
 #include "../encoding/tlv-nfd.hpp"
 #include "../encoding/encoding-buffer.hpp"
+#include "../encoding/block-helpers.hpp"
 
 namespace ndn {
 namespace nfd {