Jwplayer duration fix

JWPlayer returns video duration only after the [video starts playing](http://support.jwplayer.com/customer/portal/questions/9746437-onduration-available-only-when-video-starts-playing). Currently, popcorn doesn't recognize that, this PR attempts to fix this situation.

Might be better if everything that happens inside this `setTimeout` is moved into `getCorrectDuration`

That way the expected order the durationchange, loadedmetadata, loadeddata events fire in stays the same.

Thoughts?