Add a validation handler to check cAdvisor's underlying dependencies.

Sample output:

cAdvisor version: 0.6.2

OS version: Ubuntu 14.04.1 LTS

Kernel version: [Supported and recommended]
	Kernel version is 3.13.0-39-generic. Versions >= 2.6 are supported. 3.0+ are recommended.


Cgroup setup: [Supported and recommended]
	Available cgroups: map[cpu:1 cpuacct:1 memory:1 blkio:1 cpuset:1 devices:1 freezer:1 perf_event:1 hugetlb:1]
	Following cgroups are required: [cpu cpuacct memory]
	Following other cgroups are recommended: [blkio cpuset devices freezer]


Cgroup mount setup: [Supported and recommended]
	Cgroups are mounted at /sys/fs/cgroup.
	Any cgroup mount point that is detectible and accessible is supported. /sys/fs/cgroup is recommended as a standard location.


Docker version: [Supported and recommended]
	Docker version is 1.3.1. Versions >= 1.0 are supported. 1.2+ are recommended.


Docker driver setup: [Supported and recommended]
	Docker exec driver is native-0.2. Storage driver is devicemapper.

I didn't find any other place. Where else do we parse it?
There's one in libcontainer that we can re-use, but it hides all present-but-disabled cgroups, and only reports present and enabled ones.