Adding "fluid" mode

This is a first pass at the ideas discussed in #982.
Well, I wrote a simple page with a variety of configs when I tested this. The loaded metadata listener on the tag seemed to be firing correctly on html5 vids, and everything seemed to work. Can you tell me an embed code/options object that breaks this as written? This would really help me. If I'm not listening on the `tag` what should I be listening on?

Agreed that the dimension methods still need to be addressed. As far as terminology, the spec distinguishes between "intrinsic width" (pixels in the actual media file), "specified width" (in a dimension attribute), and "computed width/layout width" (CSS pixels on the screen). It's not impossible for all three of these to be different! For clarity's sake, I think it's really useful to adopt some solid naming conventions here in as far as variable and method names, both internally and externally. In other words, never just "width."

So should we rename the setters something like `videoCssWidth()`, since CSS is really what it's doing? E.g. the author could set a CSS width, and if he's got `* {max-width: 100%}` the rendered width might very well not be the value he just entered. Thoughts?