Fix -Wextra compiler warnings. (#184)

* Fix -Wextra compiler warnings.

Fix #183.

Signed-off-by: Kyle Fazzari <github@status.e4ward.com>

* Comment out unused parameters.

This is done everywhere possible instead of using Internals::unused().
Note that this involved moving some implementations into the
corresponding .cpp file.

Signed-off-by: Kyle Fazzari <github@status.e4ward.com>

* Fix warnings in tests as well.

Signed-off-by: Kyle Fazzari <github@status.e4ward.com>

* Leave IPv4Reassembler alone, it's growing.

Signed-off-by: Kyle Fazzari <github@status.e4ward.com>

include/tins/cxxstd.h

@@ -56,6 +56,8 @@ struct smart_ptr {
     typedef std::auto_ptr<T> type;
 #endif
 };
+
+template<class T> void unused(const T&) { }
 }
 }