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
comm can contain spaces, thus we can't use a plain scanf.

```
echo '#!/bin/cat /proc/self/stat' > 'a b'
chmod 755 a\ b
./a\ b  | od -a | head -1
0000000   1   2   8   6   6  sp   (   a  sp   b   )  sp   R  sp   1   2
```