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
Please note this is a general purpose procfs package. It's only in client_golang as I haven't found a better place right now. The interface of `/proc/$pid/fd` is to return a list of file descriptors, which was mapped here. If you want to count the file descriptors you normally use `ls -1 /proc/$pid/fd | wc -l`, which was the exact thing implemented here, leaving it to the client to do something with the result. Anyway, the shortcut is added, is this a blocker?