Adding "fluid" mode

This is a first pass at the ideas discussed in #982.
In this PR authors never touch classes directly (other than choosing a skin). My idea is that fluid mode is activated by authors either through the options config or declaratively e.g.:

    <video class="video-js vjs-default-skin" controls preload="auto" width="640" height="264"
      data-setup='{"fluid":true, "aspectRatio":"640:264"}'>

Dealing with the no-js case
-------------------------

There’s no easy pure css class solution across both the js and the no-js use cases. This is because setting the fluid class on the video embed will smush it down to zero height when js is disabled. The rules for the wrapper can’t be the same as for the solo embed element. There’s no “one class fits all”.  I see four options here:

1) Just globally add:

    .video-js {max-width: 100%}

The video element is natively responsive, so that's all it takes to get that going. It seems pretty opinionated, so I haven’t done that. And it might break some layouts where things are supposed to pop out of their container. It could work though.

2) Leave it to authors to add the above rule if they care about the no-js case? Which they should! Not sure about this, I would rather everything work straight out of the box.

3) Get fancy with CSS attribute selectors, e.g.

    video[data-setup*=‘“fluid”:true’] {max-width: 100%}

The star syntax matches any value that contains the specified string. But that’s obviously extremely brittle, because any change in spacing or quotation conventions will break the match. Bad idea.

4) Yet another more far ambitious longterm option is to break the data-setup object across multiple attributes. If we had:

    <video class="video-js vjs-default-skin" controls preload="auto" width="640" height="264"
      data-vjs-fluid=“true”
      data-vjs-aspectRatio=“640:264"
    >

Then it would be easy to select the embed code element with js off using:

    video[data-vjs-fluid*=“true”] {max-width: 100%}

This “multiple attribute” syntax is how the slideshow plugin cycle2 (and lots of frameworks) does its declarative configurations, and I think it’s brilliant. It’s a bit more typing, but it’s very legible with newlines between declarations, there's no JSON syntax to fumble, and it makes CSS attribute selectors a breeze. And the repetition gets smushed in gripping, so any bloat isn’t meaningful. This would be a big change, I realize, just want to throw this out there. I really think this is the way to go longterm.

Please advise between these four approaches?