ol.Extent as Array.<number>

This PR implements the changes to `ol.Extent` as discussed at last Thursday's meeting:

* `ol.Extent` and `ol.Rectangle` have been merged
* `ol.Extent` is now `Array.<number>`
* The order of arguments is `minX`, `maxX`, `minY`, `maxY`
* `ol.TileRange` and `ol.PixelBounds` no longer inherit from `ol.Rectangle`
* The order of arguments to `ol.TileRange` and `ol.PixelBounds` has been changed to `minX`, `maxX`, `minY`, `maxY` for consistency.

All the tests pass, and the examples seem to work except for:
* examples using Bing Maps are not tested because the API key has expired (#554)
* the zoom slider does not work because it was already broken (#553)

Please do test this, as it's a major refactoring, and it's entirely possible that I've missed something.