Update assets to jQuery-File-Upload 9.8.0

This PR includes:

* updated assets from jQuery-File-Upload [v9.8.0](https://github.com/blueimp/jQuery-File-Upload/releases/tag/9.8.0)
* moved `vendor/assets` to `app/assets` for Rails 4 compatibility
* couple more helpers for easy include `basic-plus`, `angularjs` or `jquery-ui` versions
* updated README.md