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.

Reading the comments for `onReady()`, it does sound like it can all be moved. What would be the stopping condition then? Is `if(duration == -1)` still appropriate?