Add support in the UI for uploading files to an issue
    
    This commit adds the Javascript allowing drag&drop mechanism to upload
    a file to a ticket.
    As an alternative, we also offer in the UI the possibility to use the
    more traditional `upload` button.
    All the upload are preformed directly on the fly with a progress bar
    showing up.
    
    Fixes http://209.132.184.222/progit/issue/52