Add AreaPickingEvent class

AreaPickingEvent will hold indices of the points inside in the selected area.
Update MouseEvent class to hold a new selection_mode_ attribute
Enable area picking in PCLVisualizerInteractorStyle and PointPickingCallback.

Make PCLVisualizerInteractorStyle inherit from vtkInteractorSyleRubberBandPick.
Make a room for an area picking signal and callback.
Handle the r,R switches that will enable rubber band tool.
Divert the p,P switch of vtkInteractorSyleRubberBandPick to x,X so that
PCLVisualizerInteractorStyle behavior remains unchanged.
Add an event listener for the mouse release event.
Pass the selection mode to the mouse event.
Remove vtkPointPicker header.

Adapt PointPickingCallback::Execute to the new PCLVisualizerInteractorStyle
and handle the area picking event.
Add PointPickingCallback::performAreaPick methode to return points indices
in the selected area.
Add vtkAreaPicker header.
Update PCLVisualizer to enable area picking

Use a vtkAreaPicker instead of vtkPointPicker.
Add registerAreaPickingCallback methodes.
Merge pull request #57 from nizar-sallem/area_picking

Area picking