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/util/io.hpp b/src/util/io.hpp
index 6ca861f..c61dccf 100644
--- a/src/util/io.hpp
+++ b/src/util/io.hpp
@@ -16,6 +16,7 @@
 #include "../common.hpp"
 
 #include "../encoding/block.hpp"
+#include "../encoding/buffer-stream.hpp"
 
 #include <string>
 #include <iostream>