Change stats to follow name guidelines

This is a big change to how node_exporter exports to support the guidelines and make it easier to use, and will likely break all current users.

Sample output: http://pastebin.com/jvPt9pUx

Diskstats: Split out metrics, keep 'device' label
Meminfo: Split out metrics, one each. Convert kB to bytes.
Netstats: Split out metrics, keep 'device' label.
Interrupts: Stays the same. Not perfect, but should be rarely used.
Loadavg: Make it clear it's the 1m loadavg
Last seen: Not clear this belongs in the node exporter, as it's more a user
  thing than a machine thing. Changed to absolute time rather than relative.

All stats now have appropriate counter/gauge type, and as useful docs as practical.