HTML5 placeholder support

Adds the HTML5 placeholder attribute to the token input if configured, e.g:

    $("#input").tokenInput("http://shell.loopj.com/tokeninput/tvshows.php", {
        placeholder: 'Select a TV show'
    });

Tested in Firefox 4 & Chrome. Support for older browsers can probably be given by including jquery.placeholder.js

- Makes sure the input widget gets the maximum space possible (i.e. at least the size of the input and maximum the size of the widget).
- Removing width from css as it is now set automatically.
- On the way fixes long token input not scrolling if widget width is surpassed