Add NOTICE, ALERT and EMERG log level.

This commit introduces 3 new log level. Thoses are:
    + NOTICE, which is a bit worse that INFO, but still not a warn.
    + ALERT, for case worse that critical.
    + EMERG, application is unusable.

With those 3 log levels, spdlog now has all log level accepted by
the syslog() system call.
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). Note that
the suffix _st or _mt is not present, as syslog() is thread-safe.

I also applied @gabime reviews and added license header.
Merge pull request #8 from xaqq/master

Add a syslog() based sink for Linux.
Update syslog_sink.h

Removed un needed includes