FEATURE: multithreaded approach to gather metrics from multiple nodes

- preparation for thread model
 - read configuration for munin-node hosts from file
 - construction of configuration for each thread done
- filter list of plugins according to supplied regexp
- optimize way how to get plugins from hosts (do not read it every time)
- thread aware signal handler
- handle signal for list of plugin reload
- syslog logging instead basic one
- extended documentation in README.md
make logging to syslog a flag, as opposed to the default (stderr)