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"

collector/fixtures/qdisc/results.json

@@ -0,0 +1,17 @@
+[
+    {
+        "IfaceName": "wlan0",
+        "Bytes": 42,
+        "Packets": 42,
+        "Requeues": 1,
+        "Kind": "fq",
+        "Drops": 1
+    },
+    {
+        "IfaceName": "eth0",
+        "Bytes": 83,
+        "Packets": 83,
+        "Requeues": 2,
+        "Kind": "pfifo_fast"
+    }
+]