Merge pull request #304 from chenhayat/vs2013_fix

Fix compilation error C2664  on VS2013

include/spdlog/sinks/sink.h

@@ -15,7 +15,10 @@ namespace sinks
 class sink
 {
 public:
-    sink(): _level( level::trace ) {}
+    sink()
+    {
+         _level = level::trace;
+    }
 
     virtual ~sink() {}
     virtual void log(const details::log_msg& msg) = 0;