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
The proc filesystem is commonly mounted under `/proc` (I haven't seen a system with a different mount point so far). The package methods are basically shortcuts for convenience, assuming the default is what people want. In tests (mount point is fixtures) or in the rare cases the filesystem is mounted somewhere else, people can still manually instantiate the `ProcFS` type and call `Process` on it.