Bind 'onKeyPress' during keydown instead of keyup.

Binding during keydown will be more effective at preventing certain default behavior like moving the page down as these events may happen prior to keyup. Addresses #1452

src/js/button.js

@@ -65,7 +65,7 @@ vjs.Button.prototype.onClick = function(){};
 
   // Focus - Add keyboard functionality to element
 vjs.Button.prototype.onFocus = function(){
-  vjs.on(document, 'keyup', vjs.bind(this, this.onKeyPress));
+  vjs.on(document, 'keydown', vjs.bind(this, this.onKeyPress));
 };
 
   // KeyPress (document level) - Trigger click when keys are pressed
@@ -79,5 +79,5 @@ vjs.Button.prototype.onKeyPress = function(event){
 
 // Blur - Remove keyboard triggers
 vjs.Button.prototype.onBlur = function(){
-  vjs.off(document, 'keyup', vjs.bind(this, this.onKeyPress));
+  vjs.off(document, 'keydown', vjs.bind(this, this.onKeyPress));
 };