Reduce inclusions of <boost/asio.hpp> in public headers

And especially, stop including it from the precompiled header
"common-pch.hpp". This is a necessary step in order to avoid
conflicts between net/if.h (pulled in by Asio) and linux/if.h
(required by future commits).

Change-Id: I0207be0d2be796d8ef369062a8d5195fedfef6b0
diff --git a/src/transport/stream-transport.hpp b/src/transport/stream-transport.hpp
index 5ec91b0..a0c7ca3 100644
--- a/src/transport/stream-transport.hpp
+++ b/src/transport/stream-transport.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2015 Regents of the University of California.
+ * Copyright (c) 2013-2016 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -24,6 +24,7 @@
 
 #include "transport.hpp"
 
+#include <boost/asio.hpp>
 #include <list>
 
 namespace ndn {
diff --git a/src/transport/transport.hpp b/src/transport/transport.hpp
index 663d50e..1095f14 100644
--- a/src/transport/transport.hpp
+++ b/src/transport/transport.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2014 Regents of the University of California.
+ * Copyright (c) 2013-2016 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -25,7 +25,14 @@
 #include "../common.hpp"
 #include "../encoding/block.hpp"
 
-#include <boost/asio.hpp>
+#include <boost/system/error_code.hpp>
+
+// forward declaration
+namespace boost {
+namespace asio {
+class io_service;
+} // namespace asio
+} // namespace boost
 
 namespace ndn {