Separate test example from modify example

I appreciate @ahocevar adding real data back into the modify example.  As [I mentioned](https://github.com/openlayers/ol3/pull/1807#issuecomment-37473559) in #1807, I think it would be nicer to more clearly separate the test stuff from the educational stuff.  The current modify example is [huge](https://github.com/openlayers/ol3/blob/185a655cddd63e5f7364553d52ecd1abcfb75ef5/examples/modify-features.js) and not really that pretty.
![modify](https://f.cloud.github.com/assets/41094/2475914/0eaf392a-b05d-11e3-9d97-c66725bb8121.png)

Combining things like inlining of GeoJSON and a complex styling function distracts from what I think the example should do: show people how to enable feature modification.