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.

include/spdlog/details/fast_oss.h

@@ -126,7 +126,7 @@ public:
         swap(first._dev, second._dev);
     }
 
-    std::string str()
+    std::string str() const
     {
         auto& buffer = _dev.buf();
         const char*data = buffer.data();

include/spdlog/details/spdlog_impl.h

@@ -30,6 +30,7 @@
 #include "registry.h"
 #include "../sinks/file_sinks.h"
 #include "../sinks/stdout_sinks.h"
+#include "../sinks/syslog_sink.h"
 
 inline std::shared_ptr<spdlog::logger> spdlog::get(const std::string& name)
 {
@@ -81,6 +82,13 @@ inline std::shared_ptr<spdlog::logger> spdlog::stderr_logger_st(const std::strin
 }
 
 
+// Create syslog logger
+inline std::shared_ptr<spdlog::logger> spdlog::syslog_logger(const std::string& logger_name)
+{
+    return create<spdlog::sinks::syslog_sink>(logger_name);
+}
+
+
 inline std::shared_ptr<spdlog::logger> spdlog::create(const std::string& logger_name, spdlog::sinks_init_list sinks)
 {
     return details::registry::instance().create(logger_name, sinks);