Support generating chart Javascript without <script> tag

This allows JS to be used inside a callback function, e.g. if the Google jsapi code is being loaded dynamically rather than in `<head>` you need code like:

    $.getScript('https://www.google.com/jsapi', function (data, textStatus) {
      <%= render_chart(@status_chart, 'status-chart', :script_tag => false) %>
    });