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/endian.hpp b/src/encoding/endian.hpp
index 5329112..8b330b9 100644
--- a/src/encoding/endian.hpp
+++ b/src/encoding/endian.hpp
@@ -12,17 +12,20 @@
  * @author Junxiao Shi <http://www.cs.arizona.edu/people/shijunxiao/>
  */
 
+#ifndef NDN_ENCODING_ENDIAN_HPP
+#define NDN_ENCODING_ENDIAN_HPP
+
 #ifdef __linux__
 
 #include <endian.h>
 
-#endif
+#endif // __linux__
 
 #ifdef __FreeBSD__
 
 #include <sys/endian.h>
 
-#endif
+#endif // __FreeBSD__
 
 #ifdef __APPLE__
 
@@ -40,4 +43,6 @@
 #define be64toh(x) OSSwapBigToHostInt64(x)
 #define le64toh(x) OSSwapLittleToHostInt64(x)
 
-#endif
+#endif // __APPLE__
+
+#endif // NDN_ENCODING_ENDIAN_HPP