Adding Subcontainers to cAdvisor client.

Also refactoring and improvements.
Merge pull request #264 from vishh/disk_usage

Adding filesystem stats to db storage drivers.
Merge pull request #265 from vmarmol/client

Adding Subcontainers to cAdvisor client.
Adding filesystem usage calculation for docker containers. This patch also includes some internal refactoring.
'machine' api now exports a list of all existing 'ext*' filesystems along with the capacity.