Multi-file upload

Note: a couple of JavaScript libraries are included in this branch. I have modified 3 of the files under
`zanata-war/src/main/webapp/resources/jQuery-File-Upload/9_5_6/js/`:

 - `main.js` (almost completely rewritten)
 - `jquery.fileupload-ui.js`
 - `jquery.fileupload.js`

These files are based on JQuery UI Widget architecture, where `jquery.fileupload.js` is the base widget definition, `main.js` deals with binding the widget to the specific element in the DOM, and `jquery.fileupload-ui.js` deals with UI stuff like rendering templates.

There is no need to review any other files under `zanata-war/src/main/webapp/resources` since they are just library additions.
Can we use @Slf4j?