Add diskio stats to cAdvisor.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Flatten diskio stats and use struct key.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #247 from rjnagal/diskio

Add diskio stats to cAdvisor.