Package traversal algos

[fearful-symmetry]

We might want to implement something like this:

http://software.intel.com/en-us/articles/intel-64-architecture-processor-topology-enumeration

so we can figure out how many physical processors are on a system.

[fearful-symmetry]

So far, the most readable example of this algo I've found is in the intel power_gadget source.

[fearful-symmetry]

I added a little algo to 56bd1c6

For now, I'm kind of putting this on the backburner; I'm wondering if having a separate API for multi-cpu systems would be best, or a custom type that allows the user to specify multiple devices. Something like:

NewRAPLFromThreads([]int) (RAPLHandlerArray, error)
NewRAPLDetectPackages() (RAPLHandlerArray, error)