Loading indicator

This is a proposed implementation of one of the features on the wishlist. Some notes:

  - The spinner is implemented as an [animated gif](https://github.com/mozilla/pdf.js/blob/master/web/images/loading-icon.gif), which I took from the [PDF.js example viewer](https://github.com/mozilla/pdf.js/tree/master/web)
  - It's embedded in the JS source as a base64-encoded data url.
  -  To center the spinner on each page DOM element, set the `position` style attribute of the `.plv-page-container.page-container` `divs` as `relative`. 

Feel free to merge :)