BUG: There was a problem with vtkPVFileEntry that if you opened not-first file of the series, it opened the first one. Also slider bar did not update. This fixes it. Also, closes Bug #766 - vtkPVFileEntry does not work properly when open from series

ENH: Added vtk_png.h to ensure that VTK finds the correct png.h when not using system png.  Added vtk_png_mangle.h to mangle all the public symbols in the vtkpng library to start with a vtk_ prefix.  This will avoid conflicts with applications that link to vtkIO and a separate png library.

ERR: Added explicit output stream operator to avoid overload resolution problem between operator for vtkSmartPointerBase and operator for T* on buggy compilers.

ERR: Need explicitly declared comparison operator templates to avoid ambiguous operator selection now that implicit conversion to the contained pointer has been implemented.