Add wrapX option to TileJSON source

This PR adds a `wrapX` option to `ol.source.TileJSON` in response to a [feature request](https://groups.google.com/d/msg/ol3-dev/RQpvwp_Gs5M/WJ-yflyHqHMJ) on the mailing list. This option allows disabling the wrapping of the world for TileJSON sources.