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"

end-to-end-test.sh

@@ -22,6 +22,7 @@ collectors=$(cat << COLLECTORS
   netdev
   netstat
   nfs
+  qdisc
   sockstat
   stat
   textfile
@@ -76,6 +77,7 @@ fi
   -collector.textfile.directory="collector/fixtures/textfile/two_metric_files/" \
   -collector.megacli.command="collector/fixtures/megacli" \
   -collector.wifi="collector/fixtures/wifi" \
+  -collector.qdisc="collector/fixtures/qdisc/" \
   -web.listen-address "127.0.0.1:${port}" \
   -log.level="debug" > "${tmpdir}/node_exporter.log" 2>&1 &