Make it possible to synchronize map navigation (with animation)

As the current [side-by-side example](http://ol3js.org/en/master/examples/side-by-side.html) demonstrates, using one view with two maps doesn't work for animated zoom/pan/rotate.

We should be able to provide a way for applications to have more than one map that share the rendered view state.  As the examples below demonstrate, this is possible, but it requires the use of non-exported code and some awkwardness.

This pull request is meant to serve as a discussion point for making map synchronization easier.