Added ability to specify log path, linux install scipt and made some housekeeping in docs.

[Shargann]

I needed to run exporter as systemd service, so to achieve this I added ability to specify log path. I've also included simple install script, new version of systemd service and made some housekeeping in docs to reflect changes and current state of exporter. Ah, I also added sample Grafana dashboard to use with exporter. "Sorry for my bad Go" ;) but I'm rather a sysadmin than programmer.