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
A different aspect of the greater picture I've just realized:

With the current implementation, you have to pick a different namespace for each process collector you want to register. However, if you want to monitor many processes from one binary (which was one of your original requirements, IIRC), you probably don't want a different metric name for each process. You probably want the pid in a label. That's a use-case for const labels - the `{pid="1234}` label would be constant for each process collector, and if you implement it like that, you can register multiple collectors with the same metric name. However, you have to guarantee that the pid never changes over the lifetime of the collector, so creating it in a function is a problem. And then you probably don't want a numeric pid for self (because it changes after a restart), more like `{pid="self"}` or so... Needs further discussion. Happy to do it directly once we have overlap in work time...