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
I did a benchmark on my Laptop and it takes 28ns to convert a string to a uintptr. Given that the huge majority of processes will only have tens of open FDs and that even for 100k file descriptors it takes 2ms to convert them, I'm torn whether this optimization is necessary. 

I just added `FileDescriptorsLen()` and use this in the prometheus package instead. See my last commit.