Added the GMapEventOnce class (removes the listener after the first event)

[nicolasmartin]

Hello,

This new class allows the event to be triggered just once.

It uses the 'addListenerOnce' and 'addDomListenerOnce' methods (cf. http://code.google.com/apis/maps/documentation/javascript/reference.html#event)

[laurentb]

Thanks! Merged at https://github.com/theodo/sfEasyGMapPlugin/commit/481a09169f46910b14d7b235e3f29b5c3d6684fd