Loading script in head and destroy method bugfixes

Bugfix 1:

If the script is loaded inside the `<head>` tag, the `Completer.DEFAULTS.appentTo` member will have an invalid reference, because the DOM has not been loaded yet. Implemented lazy loading to address this.

A demonstration of the bug is as follows:
```html
<!DOCTYPE html>
<html>
  <head>
    <title>jquery.textcomplete.js example</title>
    <link rel="Stylesheet" type="text/css" href="https://rawgithub.com/yuku-t/jquery-textcomplete/master/dist/jquery.textcomplete.css"/>
    <script src="https://code.jquery.com/jquery-1.9.1.min.js"></script>
    <!--
      Happens with both 0.3.3 release and master branch.

      0.3.3 release:
      <script src="https://rawgithub.com/yuku-t/jquery-textcomplete/v0.3.3/dist/jquery.textcomplete.js"></script>
    -->
    <script src="https://rawgithub.com/yuku-t/jquery-textcomplete/master/dist/jquery.textcomplete.js"></script>
    <script type="text/javascript">
      function enable() {
        $('textarea').textcomplete([{
          match: /(^|\s)(\w{2,})$/,
          search: function (term, callback) {
            var words = ['google', 'facebook', 'github', 'microsoft', 'yahoo'];
            callback($.map(words, function (word) {
              return word.indexOf(term) === 0 ? word : null;
            }));
          },
          replace: function (word) {
            return word + ' ';
          }
        }]);
      }

      function destroy() {
        $('textarea').textcomplete('destroy');
      }

      $(function () {
        $('#enable').on('click', enable);
        $('#disable').on('click', destroy);
      });
    </script>
  </head>
  <body>
    <h1>Bug</h1>
    <ol>
      <li>Type 'google', 'facebook', 'github', 'microsoft' or 'yahoo'.</li>
      <li>Click on the <strong>enable</strong> button.</li>
      <li>Type again. <span style="color:red">Bug: JavaScript exception occurs.</span></li>
    </ol>

    <textarea></textarea>
    <br />
    <button id="enable">enable</button> | <button id="disable">Disable</button>
  </body>
</html>
```