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/transport/transport.hpp b/src/transport/transport.hpp
index 8b16639..e383f4e 100644
--- a/src/transport/transport.hpp
+++ b/src/transport/transport.hpp
@@ -16,6 +16,8 @@
 #include "../common.hpp"
 #include "../encoding/block.hpp"
 
+#include <boost/asio.hpp>
+
 namespace ndn {
 
 class Transport