Make paths to pid and log configurable

I started to make the default paths `/var/run` and `/var/log` to follow unix conventions... but then I realized that would require the user to have permissions to `/var` or run rsense with sudo... Since that sounded like a bad default I opted for making the paths configurable. At some point I'll contribute to the wiki on how to configure rsense differently. Ultimately it make make sense to actually allow for a config file, but for the moment I just went with ENV variables.