Added /proc/diskstats

Read disk io information from /proc/diskstats.
This will allow the user who provides partition container hints to get partition-specific io (blkio provides io for the container, but at the disk device level).
nit: we should only need to do this compilation once. Do it in the global namespace outside this function

var partitionRegex = regexp.MustCompile("...")

also, shouldn't this in theory be [a-z]+ for those with many disks?