Add disk IO metrics to Metricbeat

[andrewkroh]

With the help of gopsutil v2.0.0 this adds disk IO metrics to Metricbeat for Linux,
Windows, and FreeBSD.

Sample event:

{
    "@timestamp": "2016-05-11T23:25:32.317Z",
    "beat": {
        "hostname": "5ff05546664c",
        "name": "5ff05546664c"
    },
    "metricset": "disk",
    "module": "system",
    "rtt": 17082,
    "system-disk": {
        "io_time": 21110,
        "name": "sda2",
        "read_bytes": 160079872,
        "read_count": 21292,
        "read_time": 20070,
        "write_bytes": 799186944,
        "write_count": 3208,
        "write_time": 90070
    },
    "type": "metricsets"
}

[ruflin]

Awesome :-)