Adds scrollable-div example and functionality.  Fixes bug with lingering styles after header is no longer sticky

I was using this in a layout where the entire page was scrolling, and I had long tables containing lots of data.  I changed the layout so the content div is absolutely positioned and scrollable.  The table headers stopped being sticky.  I modified the plugin to take a 'scrollableArea' option, which you can populate with the DOM element that will be scrolling (if you're scrolling an element other than the window.

Another example was added.  The tables have variable width in the example.

Also, table header cells still contained the 'max-width' and 'min-width' properties that were set for them while the header was sticky.  I modified this behavior so the cell widths reset to the inline max/min-width values they had before the header became sticky.