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
Add option to process collector to pass pid funciton

In order to allow clients to export metrics for another process (for
example in an exporter example like haproxy_exporter collecting metrics
about running haproxy instancs), this change set adds the option to pass
a pid function instead of a single integer. Therefore, clients can pass
a differen pid after a restart of the monitored service.