Native Munin poller

This patch adds support for native Munin polling via remote munin-node agent.

It's goal is to save people who already use Munin on many servers from having to change the remote agent. For new installations, however, observium agent is, of course, better choice.

munin-node poller implements only the data retrieval from a remote munin-node (no SSL support yet) and relies on unix-agent's original munin code for data handling and storage. Munin graphs appear as if they were retrieved via unix-agent.