Add API version v1.2 with /docker endpoint.

The /docker endpoint lists all Docker containers under one unified
namespace.
Add v1.2 documentation to the README.
Merge pull request #278 from vmarmol/docker-api

Add API version v1.2 with /docker endpoint.
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).
Gofmt all files