Added support for multiple music files

Allow up to 4 music files to be used for conversion. This allows the web
program to maintain a previous and next "on deck" to support smoother
playback without having to convert an entire playlist.
New Audio class, implemented Next/Previous

Created a new Audio class (audio.js) to combine functionality from HTML5
audio node and jQuery. Moved all audio access inside Player module; this
module adds event handlers and some additional control over audio node.

playlist.js now implements methods for next/prev, including support for
future continuous play functionality.
Updates to layout/styling