FEATURE: add support for remote node configuration

- any munin-node could support remote or indirect fetching of plugin's data
- add support for munin-node:remote-node syntax of host parameter
- add process logic to ask for remote node data
- displayname should be used to correctly identify such metrics
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