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
Chatted with @grobie - Linux doesn't actually provide any API (or we were too stupid to find it) for getting *just* the count. So maybe this is what we need to do for now.

The first answer on this post:

http://stackoverflow.com/questions/7976769/getting-count-of-current-used-file-descriptors-from-c-code

...also gives an interesting option, but I'm not sure if iterating from 0 -> MAXFDS is a better option than what we currently have here.