Support for Graylog

[amotl]

Hi there,

for supporting Graylog's Geolocation data as the source, @jkavan contributed grafana#276 the other day. It would be nice to have this feature also in this code base. See also grafana#41.

With kind regards,
Andreas.

[jkavan]

I should be able to do this at some point. Can't promise a timeline, though.