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/tlv.hpp b/src/encoding/tlv.hpp
index 594cdce..d071a2e 100644
--- a/src/encoding/tlv.hpp
+++ b/src/encoding/tlv.hpp
@@ -10,8 +10,8 @@
  * ndn-cxx library copyright, permissions, and redistribution restrictions.
  */
 
-#ifndef NDN_TLV_HPP
-#define NDN_TLV_HPP
+#ifndef NDN_ENCODING_TLV_HPP
+#define NDN_ENCODING_TLV_HPP
 
 #include <stdexcept>
 #include <iterator>
@@ -527,4 +527,4 @@
 
 } // namespace ndn
 
-#endif // NDN_TLV_HPP
+#endif // NDN_ENCODING_TLV_HPP