Mounted partitions space usage metrics per container

This computes the space usage for mounted partitions. It takes in a list of mounted partitions from containerHints
and computes the device's disk usage(so each mount must be a separate partition). This is useful for users who
mount partitions on containers and store most of the container's persistent data on those partitions.
code review fixes
Renamed mount and removed dead code
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
Put client inside its own package

This will let me do things like import "github.com/google/cadvisor/client"
without needing to include all of cadvisor in my project
Merge pull request #275 from kevin1024/client-package

Put client inside its own package
Flatten ContainerStats struct
Document client
Made changes to client docs as recommended by @vmarmol
Merge pull request #276 from kevin1024/document-client

Document client
code review fixes
gofmt all files
Add API version v1.2 with /docker endpoint.

The /docker endpoint lists all Docker containers under one unified
namespace.
Merge pull request #274 from ashahab-altiscale/raw-disk-space

Mounted partitions space usage metrics per container
Add v1.2 documentation to the README.
Merge pull request #278 from vmarmol/docker-api

Add API version v1.2 with /docker endpoint.
Additional blkio stats

This adds blkio.io_wait_time, blkio.io_service_time, blkio.io_merged, and blkio.time to the api
Output correct content-type with API responses.

Fixes #281
Merge pull request #279 from ashahab-altiscale/additional-disk-stats

Added /proc/diskstats