Bug fixes and minor features added

Hello David, there was a bug in my code which was causing some troubles (the fix is included). I have also added the possibility to change the size of the widget's handles. Compilation should be ok now for both VTK<=5.x and VTK=6.x (I'm currently using vtk 5.10.1). I have also removed the Example_Widget since did not make much sense alone. Finally I added some lines in the README file.