add diskstats on Darwin (#593)

* Add diskstats collector for Darwin

* Update year in the header

* Update README.md

* Add github.com/lufia/iostat to vendored packages

* Change stats to follow naming guidelines

* Add a entry of github.com/lufia/iostat into vendor.json

* Remove /proc/diskstats from description

vendor/github.com/lufia/iostat/iostat_darwin.go

@@ -0,0 +1,37 @@
+// +build darwin
+
+package iostat
+
+// #cgo LDFLAGS: -framework CoreFoundation -framework IOKit
+// #include <stdint.h>
+// #include "iostat_darwin.h"
+import "C"
+import (
+	"time"
+)
+
+// ReadDriveStats returns statictics of each of the drives.
+func ReadDriveStats() ([]*DriveStats, error) {
+	var buf [C.NDRIVE]C.DriveStats
+	n, err := C.readdrivestat(&buf[0], C.int(len(buf)))
+	if err != nil {
+		return nil, err
+	}
+	stats := make([]*DriveStats, n)
+	for i := 0; i < int(n); i++ {
+		stats[i] = &DriveStats{
+			Name:           C.GoString(&buf[i].name[0]),
+			Size:           int64(buf[i].size),
+			BlockSize:      int64(buf[i].blocksize),
+			BytesRead:      int64(buf[i].read),
+			BytesWritten:   int64(buf[i].written),
+			NumRead:        int64(buf[i].nread),
+			NumWrite:       int64(buf[i].nwrite),
+			TotalReadTime:  time.Duration(buf[i].readtime),
+			TotalWriteTime: time.Duration(buf[i].writetime),
+			ReadLatency:    time.Duration(buf[i].readlat),
+			WriteLatency:   time.Duration(buf[i].writelat),
+		}
+	}
+	return stats, nil
+}