Add COPYING.txt with public domain dedication

Change "license" field in package.json and bower.json to:

"license" : "SEE LICENSE IN COPYING.txt"

to avoid NPM warning.

COPYING.txt

@@ -0,0 +1,9 @@
+Public Domain
+
+The person who associated a work with this deed has dedicated the work to the
+public domain by waiving all of his or her rights to the work worldwide under
+copyright law, including all related and neighboring rights, to the extent
+allowed by law.
+
+You can copy, modify, distribute and perform the work, even for commercial
+purposes, all without asking permission.

bower.json

@@ -1,6 +1,6 @@
 {
   "name": "tweetnacl",
-  "version": "0.14.0",
+  "version": "0.14.1",
   "homepage": "https://tweetnacl.js.org",
   "authors": [
     "TweetNaCl.js Contributors"
@@ -25,7 +25,7 @@
     "salsa20",
     "signatures"
   ],
-  "license": "Public domain",
+  "license": "SEE LICENSE IN COPYING.txt",
   "ignore": [
     "**/.*",
     "node_modules",

package.json

@@ -1,6 +1,6 @@
 {
   "name": "tweetnacl",
-  "version": "0.14.0",
+  "version": "0.14.1",
   "description": "Port of TweetNaCl cryptographic library to JavaScript",
   "main": "nacl-fast.js",
   "directories": {
@@ -35,7 +35,7 @@
     "signatures"
   ],
   "author": "TweetNaCl-js contributors",
-  "license": "Public domain",
+  "license": "SEE LICENSE IN COPYING.txt",
   "bugs": {
     "url": "https://github.com/dchest/tweetnacl-js/issues"
   },