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
@bbrazil Have a look at `NewProcessCollector()` ([line 55 atm](https://github.com/prometheus/client_golang/pull/36/files#diff-283845bac7bc2f9901269f63a8dd6db5R54)). It uses a noopCollector by default (returns immediately) and only uses procfs if it it's available. Therefore, on non-Linux systems the collector won't emit any metrics and not cause any issues or overhead. Is this fine with you?

In addition to the procfsCollect method, I envision addition collect implementations for other systems eventually.