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/Makefile

@@ -0,0 +1,11 @@
+build:
+	go fmt
+	go build
+	go vet
+	staticcheck
+	#golint -set_exit_status
+	go test -v -race -tags=integration
+
+cover:
+	go test -coverprofile=coverage.out
+	go tool cover -html=coverage.out