3808 fiducial picking

Note this branch is for testing and should not be merged as-is, but it can be built for testing if needed.

This version depends on changes in Slicer/VTK [1].

[1] https://github.com/pieper/VTK/compare/Slicer:slicer-v6.2.0-7167ad1_2014-08-21_v2...3808-fiducial-picking

If all looks good, merge the VTK changes and then update SuperBuild/External_VTKv6.cmake to point to the right place before committing to slicer svn.