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_linux.go

@@ -0,0 +1,12 @@
+// +build !darwin
+
+package iostat
+
+import (
+	"errors"
+)
+
+// ReadDriveStats returns statictics of each of the drives.
+func ReadDriveStats() ([]*DriveStats, error) {
+	return nil, errors.New("not implement")
+}