Generate key_worker.js

Instead of calling importScripts, which is prone to relative path
issues, generate the worker script with everything it needs included.

Gruntfile.js

@@ -60,6 +60,16 @@ module.exports = function(grunt) {
         ],
         dest: 'js/libtextsecure.js',
       },
+      key_worker: {
+        options: {
+          banner: 'var window = this;\n',
+        },
+        src: [
+          'js/libtextsecure.js',
+          'libtextsecure/key_worker.js'
+        ],
+        dest: 'js/key_worker.js'
+      },
       libtextsecuretest: {
         src: [
           'components/mocha/mocha.js',
@@ -152,6 +162,10 @@ module.exports = function(grunt) {
         files: ['./libtextsecure/*.js', './libtextsecure/storage/*.js'],
         tasks: ['concat:libtextsecure']
       },
+      key_worker: {
+        files: ['<%= concat.key_worker.src %>'],
+        tasks: ['concat:key_worker']
+      },
       dist: {
         files: ['<%= dist.src %>'],
         tasks: ['copy']