clean up unused nats code

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

vendor/github.com/pmezard/go-difflib/README.md

@@ -0,0 +1,50 @@
+go-difflib
+==========
+
+[![Build Status](https://travis-ci.org/pmezard/go-difflib.png?branch=master)](https://travis-ci.org/pmezard/go-difflib)
+[![GoDoc](https://godoc.org/github.com/pmezard/go-difflib/difflib?status.svg)](https://godoc.org/github.com/pmezard/go-difflib/difflib)
+
+Go-difflib is a partial port of python 3 difflib package. Its main goal
+was to make unified and context diff available in pure Go, mostly for
+testing purposes.
+
+The following class and functions (and related tests) have be ported:
+
+* `SequenceMatcher`
+* `unified_diff()`
+* `context_diff()`
+
+## Installation
+
+```bash
+$ go get github.com/pmezard/go-difflib/difflib
+```
+
+### Quick Start
+
+Diffs are configured with Unified (or ContextDiff) structures, and can
+be output to an io.Writer or returned as a string.
+
+```Go
+diff := UnifiedDiff{
+    A:        difflib.SplitLines("foo\nbar\n"),
+    B:        difflib.SplitLines("foo\nbaz\n"),
+    FromFile: "Original",
+    ToFile:   "Current",
+    Context:  3,
+}
+text, _ := GetUnifiedDiffString(diff)
+fmt.Printf(text)
+```
+
+would output:
+
+```
+--- Original
++++ Current
+@@ -1,3 +1,3 @@
+ foo
+-bar
++baz
+```
+