Merge pull request #7 from xaqq/master

Add NOTICE, ALERT and EMERG log level - so spdlog log levels could be matched to syslog's
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). Not that
_st or _mt is not present, as syslog() is thread-safe.