Alert syslog

As per https://github.com/etsy/skyline/pull/88

Added new alerter syslog
alert_syslog to write anomalous metrics to the syslog at LOG_LOCAL4, with
Anomalous metric: %s (value: %s) so LOG_WARN priority this creates in local log
and ships to any remote syslog as well, so that it can be used further down a
data pipeline in elasticsearch, riemann, etc
Modified:
readme.md
src/settings.py.example
src/analyzer/alerters.py