Using statements have been added to XdmfSharedPtr.hpp. All used of
boost::shared_ptr and boost::shared_dynamic_cast have been replaced
by shared_ptr and shared_dynamic_cast. Slight modification to XdmfCore.i
to get swig wrapping working again.

Hopefully this will become a standard style for Xdmf development.
All spaces - 80 character line wrapping.

ENH: Rename all subclasses of XdmfGrid to more syntactically friendly names: i.e. XdmfGridUnstructured to XdmfUnstructuredGrid.

ENH: Cleanup comments and inheritance syntax a bit.  Remove unnecessary Geometry Types not needed since splitting structured and unstructured grids
into separate hierarchies.

ENH: Added XdmfGridUnstructured and turned XdmfGrid into an abstract base class for all grids (both structured and unstructured).  Modified a bunch of utils and other classes
to fit this new hierarchy.