Support for AMD usage of jwplayer (require js)

使用require js 模块化代码时，其中播放器用的是jwplayer7.x 然后载入jwplayer.js后总是报license无效（license已经加入），最后在jwplayer官网论坛里找到问题：

The reason seems to be that the flash player needs access to window.jwplayer. window.jwplayer is not set when using AMD (or CommonJS), therefore the flash player doesn't work.

While waiting for a fix, a workaround is to simply set window.jwplayer in the app's code:

define(['jwplayer'], function (jwplayer) {
  window.jwplayer = jwplayer;

  ...
})