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.
Useless type annotation. The compiler already knows the type, because it knows the signature of the $parse function.