Revert "Use protocol buffer module for libtextsecure"

This does not work with `ArrayBuffer` out of the box and will need to be
investigated and tested more before switching.

Some APIs also seemed to have changed:
- Before (OOP): `new textsecure.protobuf.WebSocketMessage(…).encode();`
- After (functional):
  ```
  textsecure.protobuf.WebSocketMessage.encode(
    new textsecure.protobuf.WebSocketMessage(…)
  );
  ```

This reverts commit d758119d9de4f99742ea941e86180b1d600c2f22.

Gruntfile.js

@@ -56,6 +56,7 @@ module.exports = function(grunt) {
           'libtextsecure/storage/user.js',
           'libtextsecure/storage/groups.js',
           'libtextsecure/storage/unprocessed.js',
+          'libtextsecure/protobufs.js',
           'libtextsecure/helpers.js',
           'libtextsecure/stringview.js',
           'libtextsecure/event_target.js',