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 How do you envision the API?

Option 1: I'm fine with the default prometheus gives me and use the helper methods defined on the package.

```go
// register something
prometheus.MustRegister(myMetric)

// get handler
prometheus.Handler()
```

Option 2: I'm a power user and want full control about everything.

```go
// get custom registry without anything pre-configured/registered
r := prometheus.NewRegistry()

// register something
r.MustRegister(myMetric)

// get handler
r.Handler()
```

This would follow the design of other go stdlibs like the [net/http package](http://godoc.org/net/http). I wouldn't have broadened the client API even more by adding `prometheus.RegisterStandardExports()` and `prometheus.UnregisterStandardExports()` methods personally, but happy to follow what ever we agree on. Comments @u-c-l @juliusv ?