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.go

@@ -0,0 +1,20 @@
+// Package iostat presents I/O statistics.
+package iostat
+
+import "time"
+
+// DriveStats represents I/O statistics of a drive.
+type DriveStats struct {
+	Name      string // drive name
+	Size      int64  // total drive size in bytes
+	BlockSize int64  // block size in bytes
+
+	BytesRead      int64
+	BytesWritten   int64
+	NumRead        int64
+	NumWrite       int64
+	TotalReadTime  time.Duration
+	TotalWriteTime time.Duration
+	ReadLatency    time.Duration
+	WriteLatency   time.Duration
+}