Add support to create pidfile & move processing to background

I'm working on adding init scripts (*NIX) for riemann-tools and noticed the lack of PID File creation.  Additionally, I needed to move the riemann-tools processes to the background to allow the process to be daemonized.

- Add support to automatically create a pid file 
- Moved a few riemann-tools processes (HAProxy, Riak, health) to process in the background;