Responsive tables refactored as a plugin, for #13

Responsive Table is now a regular jQuery plugin: `$('table').reponsiveTable()`.

The jQueryUI Widget library is no longer needed and has been removed. 

I also removed the auto-initialization from responsive-table.js (which ran it against all `<table>`s) so that devs may pass in options and choose which tables to apply it to. This will be updated in the wiki.

The file has been renamed to `responsive-table.js` (no 's' at the end) for consistency with the plugin name and the naming pattern that most jQuery plugins follow.