Add emscripten-compiled curve25519 module

Build with `grunt compile && grunt concat:curve25519` after installing
emscripten.

Enable by either (a) not loading nativeclient.js or (b) setting
`textsecure.NATIVE_CLIENT = false` before loading nativeclient.js.

test/crypto_test.js

@@ -15,6 +15,7 @@
  */
 
 'use strict';
+window.assert = chai.assert;
 
 describe("Crypto", function() {
     describe("Encrypt AES-CBC", function() {