Adding "fluid" mode

This is a first pass at the ideas discussed in #982.
I should get clarification on something I think. In your PR, can a user just add the classes, or do they have to still set the fluid and aspectRatio options? In issue #982 there was an "All CSS" use case, where the user would just have to add css classes to the player to get a fluid layout.

In that use case I think it's cleaner to allow the user to just have to add a single class, `vjs-fluid`, to get a fluid player, and we're asking for bug reports if we have a user-added class that breaks the player on its own.