Fix clicktracker trackURLs call; clean up events; etc.

* Add `showSkipButton` option
* Fix `<ClickTracking>` trackURLs call (it expects an array)
* Trigger an `adclick` event when the ad is clicked
* Add `contentupdate` comment
* Export companion ad via `player.vast.companion`
* Unhook events when the ad ends
* Add error tracking
* Trigger the `complete` event when the ad ends
* Add support for multiple sources of the same type to `createSourceObjects`