Added reference to container in ResumableFile & improved browsers compatability

I'd like to suggest some improvements:

- If multiple files are uploaded, it is important to show uploading status and progress in div, which is linked to container where uploaded file was selected. So, I suggest to add a property 'container' to the ResumableFile object.

- File dialog won't open on some versions of Opera and Firefox and other browsers because of they don't allow triggering "click" on hidden elements. I suggest to trigger click event with additional actions: set opacity to 0, show, focus, click, and then hide.