Moving from IDs to Classes to allow multiple BigVideos

I had a need to have multiple BigVideos on one page at a time. I'm targeting elements rather than the background of the page, and using BigVideo to handle dynamic resizing of the video with the browser.

This change allows you to pass in an optional, unique 'videoNum' with each video you create so they don't conflict with each other.

Some of the files modes got changed as well for some reason, I can make a new pull request if that's an issue.