Limits pointer event disabling to YouTube videos alone

When using this plugin on the same page as a native HTML5 video, the YouTube plugin sets certain styles that affect certain features of the native player. For example, the big play button no longer gives a pointer for HTML5 videos. (And in some instances, like with audio elements, cannot be clicked on, preventing playback.)

Compare the [default behavior](http://jsfiddle.net/788bE/1/) with the [behavior when the YouTube plugin is present](http://jsfiddle.net/788bE/).

This pull request specifies that these CSS properties only apply to YouTube videos, rather than all Video.js instances.