Add qdisc collector for Linux (#580)

* Add qdisc collector for Linux

This collector gathers basic queueing discipline metrics via netlink,
similarly to what `tc -s qdisc show` does.

* qdisc collector: nl-specific code moved, names fixed

- netlink-specific parts moved to github.com/ema/qdisc
- avoid using shortened names
- counters renamed into XXX_total

* Get rid of parseMessage error checking leftover

* Add github.com/ema/qdisc to vendored packages

* Update help texts and comments

* Add qdisc collector to README file

* qdisc collector end-to-end testing

* Update qdisc dependency to latest version

Update github.com/ema/qdisc dependency to revision 2c7e72d, which
includes unit testing.

* qdisc collector: rename "iface" label into "device"

vendor/github.com/ema/qdisc/README.md

@@ -0,0 +1,26 @@
+qdisc [![Build Status](https://travis-ci.org/ema/qdisc.svg?branch=master)](https://travis-ci.org/ema/qdisc)
+=====
+
+Package `qdisc` allows to get queuing discipline information via netlink,
+similarly to what `tc -s qdisc show` does.
+
+Example usage
+-------------
+
+    package main
+
+    import (
+        "fmt"
+
+        "github.com/ema/qdisc"
+    )
+
+    func main() {
+        info, err := qdisc.Get()
+
+        if err == nil {
+            for _, msg := range info {
+                fmt.Printf("%+v\n", msg)
+            }
+        }
+    }