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/curve25519_compiled_test.js

@@ -0,0 +1,28 @@
+/* vim: ts=4:sw=4
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+'use strict';
+
+
+describe('curve25519_compiled.js', function() {
+    describe('curve25519_donna', function() {
+        it('exists', function() {
+            var curve25519_donna = Module.cwrap('curve25519_sign', 'string', 'string');
+            assert.isDefined(Module.cwrap);
+        });
+    });
+    test_curve25519_implementation(curve25519);
+});