Support JW Player 6 responsive layout (% based width and aspect ratio)

Problem/Motivation

JW Player 6 support responsive layout (see http://www.longtailvideo.com/support/jw-player/32427/responsive-design-s...). This is done by setting a percentage based width and an aspect ratio instead of pixels based width and height.

Proposed resolution

Add support for responsive layout in presets. When the responsive layout is selected, the width field should be threaded as % and the height field should be replaced by an aspect ratio one. Ideally known aspect ratio should be offered in a select. A 'custom' option should be provided, the custom option would add a textfield to enter a custom value.