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
According to proc(5) and scan(3) `Comm` is a squence of non-whitespace characters.

> comm %s     (2) The filename of the executable, in parentheses.  This is visible whether or not the executable is swapped out.

-- proc(5)

> s      Matches a sequence of non-white-space characters; the next
              pointer must be a pointer to character array that is long
              enough to hold the input sequence and the terminating null
              byte ('\0'), which is added automatically.  The input string
              stops at white space or at the maximum field width, whichever
              occurs first.

-- scanf(3)

I guess what you expect here is the cmdline, which is available under `/proc/$pid/cmdline`.