Fix e2e tests issue 131.

Squashed & Rebased of #161.

Change log:
- `main.po.js` is now a template file.
- Made `protractor` depends on `watch` & `protractor:dist` depends on `build`, fix tasks dependencies. Removed `gulp protractor-only` from test.
True, I just lazily copied from the previous solution, but should be fixed into something like this, updating now.