Add groupinteraction example

This PR provides an example on how we could use angular directives to manage a button group that toggles exclusive *ol3* interactions.

Each toggle button is managed by a *goBtn* directive and is binded to an interaction.active property.
The *goBtnGroup* controller manage an array of *goBtn* scopes, and assumes that only one interaction is active at the same time.
We need to decide whether we use angular.forEach or goog.array.forEach. I'd be tempted to use the latter, because the Compiler probably renames goog.array.forEach to something short, removing the namespaces.