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 a 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.