Standard exports

The procfs package can be used to query the pseudo-filesystem commonly
mounted under /proc. It currently provides information from /proc/stat,
/proc/$pid/stat, /proc/$pid/fd and /proc/$pid/limits.

This change adds two new collectors to the prometheus package which
export metrics about a given or the current process.

* ProcessCollector exports metrics about cpu time, vss, rss, fd usage as
  well as the start time of a given process.
* GoCollector exports currently only the number of active goroutines.

Both collectors are active by default.

Background: https://docs.google.com/document/d/1Q0MXWdwp1mdXCzNRak6bW5LLVylVRXhdi7_21Sg15xQ/edit?usp=sharing
Making the `Registry` public largely reduces side-effects on client code. By making the client pass in its own `Registry` instead of a relying on a global one, testing becomes so much easier.

I suggest a small change over @grobie's second option (which I obviously favour). The `Registry` type itself could implement the `http.Handler` interface. So no need to implement a `Handler` method.