Add procfs package

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.
Add ProcessCollector and GoCollector

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.
Register process and go collectors by default
Handle proc stat parsing of edge cases

Fix parsing of stat for processes with command names including
parenthesis or spaces.
Add shortcut to export number of open file descriptors