Btngroup

This PR proposes to integrate the `goBtnGroup` and `goBtn` directives into the core of ngeo.
Those directive were introduced in *interactionbtngroup* example.

It also improves a bit those directives :
- add a false default value if the ngModel associated to the goBtn is undefined
- allow to have a goBtn outside a goBtnGroup directive (make the goBtnGroup controller optional)
- provide a new example