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/vendor.json

@@ -26,6 +26,12 @@
 			"revision": "e97b35f834b17eaa82afe3d44715c34736bfa12b",
 			"revisionTime": "2017-02-01T10:47:36Z"
 		},
+		{
+			"checksumSHA1": "5tGizHomeMFHcyROMD2ntrKFR1Q=",
+			"path": "github.com/ema/qdisc",
+			"revision": "2c7e72d2f110bdddf17026872f6c396a9e1e4809",
+			"revisionTime": "2017-05-16T10:52:09Z"
+		},
 		{
 			"checksumSHA1": "Yim1r9qkEFFLLH8JngB4kZTLsYI=",
 			"path": "github.com/godbus/dbus",