Refactoring/misc. improvements

[veltman]

• If worker option is truthy, require Redis, localhost by default
• Automatically compute the lineHeight of a caption from the font definition
• Normalize audio volume when rendering the video
• More tests
• Eliminate jQuery dependency - only being used for multipart AJAX submission and $.extend(), the latter could be replaced with underscore and the former could be a vanilla XHR.
• Rather than sequential queue with callbacks, it might be a little more legible for the various rendering steps to be a chain of promises.