Change stats to follow name guidelines.

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.
Expand docs per code review.
Trim down a bit per feedback.
Merge pull request #11 from brian-brazil/native-labels

Change stats to follow name guidelines