New OverviewMap control

This PR contains 1 new OverviewMap demonstrated in 1 new example, and there's also an other example showing how to create a custom overview map using a second map that follows the first one.

This is my first OL3 PR, so I'm pretty sure there are concepts I haven't understand yet that would require changes in code and more other stuff, such as 'tests' I suppose.

Do not hesitate to comment.  Thanks to you all who helped me.