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.
I'm concerned with the renaming of the ngModelSet property here. The compiler could potentially use different names here and in the controller's activate function.