Issue #12: Add background collection task and rework the value plugin a bit more.
Rename periodic_cmd to value_cmd in preparation of creating a table_cmd plugin.
Get rid of stubs.
Fix include_dir config key.
Shorten the log line a bit by using module name instead of filename.
Issue #12: Implement snmp tables.
Convert value_cmd to new plugin data model.
Issue #12: Convert disk_utilization plugin to AgentX snmpy.