Add a syslog() based sink for Linux.

This commit introduce a new sink: syslog_sink.
This sink is Linux only, and will write log entries to the system
logger, using the syslog() library call.

It is instanciable using spdlog::syslog_logger(name). Note that
the suffix _st or _mt is not present, as syslog() is thread-safe.

I also applied @gabime reviews and added license header.

example/example.cpp

@@ -64,6 +64,10 @@ int main(int, char* [])
         SPDLOG_TRACE(file_logger, "This is a trace message (only #ifdef _DEBUG)", 123);
 
 
+	#ifdef __linux__
+	auto syslog_logger = spd::syslog_logger("syslog");
+	syslog_logger->warn("This is warning that will end up in syslog. This is Linux only!");
+	#endif
     }
     catch (const spd::spdlog_ex& ex)
     {