Add missing include and forward declaration.

Forward declaration of vtkRenderWindow was missing in pcl_plotter.h.
Also it is not sufficient to forward declare vtkSmartPointer, the
corresponding header shound be included.
Merge pull request #260 from taketwo/fix-plotter-includes

Add missing include and forward declaration in pcl_plotter.h
Fixed documentation for removeNaNFromPointCloud().

The old (misleading) documentation was copied over from another overload
of this function.
Merge pull request #261 from taketwo/fix-documentation

Fixed documentation for removeNaNFromPointCloud()