Refactor node_exporter

Hi,

I've refactored the node exporter by:
- moving the exporter portion to main
- making listen and interval flags instead of config (the config is only used for the attributes now)
- renamed exporter/ -> collector/

I've also added some tests for the native exporter.
Hope it's right to base this against my add-more-metrics branch. Anyways please review both PRs and I'll figure out how to merge them.