Vertical slider

* Change vjs.findPosX to vjs.findPosition to return both left and top. 

This also removes the fallback method that doesn't use the getBoundingClientRect because according to [MDN](https://developer.mozilla.org/en-US/docs/DOM/element.getBoundingClientRect#Browser_compatibility) and [PPK](http://www.quirksmode.org/dom/w3c_cssom.html#documentview), it should be supported just about everywhere.

* Make sliders able to handle vertical sliders in calculateDistance. 

This takes a 'vertical' option on the slider bar itself. If set, it will use the top and pageY values to calculate the slider positions.

For example, to make a vertical volume slider:
```javascript
  var player = videojs(document.querySelector('.video-js'), {
    'children': {
      'controlBar': {
        'children': {
          'volumeControl': {
            'children': {
              'volumeBar' : {
                vertical: true
              }
            }
          }
        }
      }
    }
  });
```