does not checkScroll() when plugin is already removed from element (after remove.ScrollToFixed was triggered)

Hello, it is nice to work on your plugin again )))

See the sample below. The problem this PR tries to solve is a correct removal of a plugin. Without the line the PR adds, after plugin is removed by ```trigger('remove.ScrollToFixed')```, the ```checkScroll``` is called (I have one run in my sample) which causes additional malicious ```setFixed()``` call on elements which should stay on their "unfixed" original positions. This all seems strange, because needed ```.unbind``` calls are there (in ```bind('remove.ScrollToFixed')```), but anyway I have exactly one call of ```checkScroll``` which makes markers "fixed" again.

```html
<!DOCTYPE html>
<html>
<head>

  <meta charset='utf-8'>
  <title> *** ScrollToFixed with endless scroll ***</title>

  <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js" type="text/javascript"></script>
  <script src="https://raw.github.com/stanislaw/ScrollToFixed/master/jquery-scrolltofixed.js" type="text/javascript"></script>
  <script src="https://raw.github.com/BorisMoore/jsrender/master/jsrender.js" type="text/javascript"></script>

  <style type="text/css">
    body {
      margin-top:10px;
    }

    .wrapper {
      width: 500px;
      margin: 0 auto;
    }

    .marker {
      position: absolute; // This is important
      margin: 0 auto;
      width: 200px;
      height: 98px;
      border: 3px dashed red;
      text-align: center;
      font-size: 20px;
    }

    .section {
      margin: 0;
      text-align: center;
      font-size: 40px;
      height: 800px;
      border: 3px #666 dashed;
    }
  </style>

  <script>
    $(function() {
      function addContainers() {
        var nextN = $('.marker-container').length + 1;
        var containers = []

        // Each container has 2 sections now
        for(var i = nextN; i <= nextN + 0; i++) {
          containers.push({ number: i})
        };

        $(".wrapper").html($(".wrapper").html() + $("#template").render(containers));
      }

      function registerMarkers() {
        var collection = $('.marker');

        $.each(collection, function(index, marker) {
          // The last element always does not have a limit
          if (index == collection.length - 1) {
            if ($.isScrollToFixed(marker)) $(marker).trigger('remove.ScrollToFixed');
            $(marker).scrollToFixed();
          } else {
            var nextMarker = $(collection[index + 1]);

            // Skip current, if it and the following one are already registered
            if ($.isScrollToFixed(marker) && $.isScrollToFixed(nextMarker)) return true;
            // On this stage we have all unregistered markers and the last registered one, which precedes the first unregistered marker.

            var nextMarkerC = nextMarker.parent(); // For offsets we rely on stable position of marker container

            $(marker).scrollToFixed({
              limit: nextMarkerC.offset().top - 100
            });
          }
        });
      }

      addContainers();
      registerMarkers();

      $(window).bind('scroll', function() {
        // If we reached the end of window
        if (document.body.scrollHeight - $(this).scrollTop() <= $(this).height()) {
          addContainers();
          registerMarkers();
        }
      });

      // <= 600 is a check for "responsive design"
      $(window).unbind('resize.scrollingDateMarkers').bind('resize.scrollingDateMarkers', function(evt) {
        var $collection = $('.marker');
        if ($(window).width() <= 600) {
          if ($.isScrollToFixed($collection)) $($collection).trigger('remove.ScrollToFixed');
        } else {
          if (!$.isScrollToFixed($collection)) registerMarkers;
        }
      });

    });
  </script>
</head>
<body>

  <script id="template" type="text/x-jsrender">
    <div class="marker-container">
      <div class="marker">Date {{>number}}</div>
    </div>
    <div class="section">One event</div>
    <div class="section">Another event</div>
  </script>

  <div class="wrapper"></div>
</body>
</html>
```