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/encoding/oid.cpp b/src/encoding/oid.cpp
index edbfa1a..f9085b2 100644
--- a/src/encoding/oid.cpp
+++ b/src/encoding/oid.cpp
@@ -17,6 +17,8 @@
 
 #include "../security/cryptopp.hpp"
 
+#include <sstream>
+
 using namespace std;
 using namespace CryptoPP;