Added support for Flask S3 extension

In response to [Issue #45](https://github.com/miracle2k/flask-assets/issues/45), this commit adds support for use with the [Flask S3](https://github.com/e-dard/flask-s3) extension.

To use, add a config variable `FLASK_ASSETS_USE_S3` and set it to `True`, for example

   app.config['FLASK_ASSETS_USE_S3'] = True

A full example can be seen [here](https://github.com/eriktaubeneck/flask-s3-assets-example).

Small caveat: Anytime you change a bundle, you must run something to the effect of

    from app import app
    import flask_s3
    flask_s3.create_all(app)

in order for the new changes to be pushed to S3. It is also recommended to use versioning within the filename rather than the query string when using with CloudFront, (you can tell CloutFront to recognize unique query strings, but it's not recommended), as is done in the above example. Also note that if you want to use CloutFront, you will need to install Flask S3 from [my fork](https://github.com/eriktaubeneck/flask-s3) until this [pull request](https://github.com/e-dard/flask-s3/pull/8) is merged in.