Use absolute import for OCCViewer; fix an inheritance issue

This fixes issue #25
Fixed pickling test for TopoDS shapes
Added missing DrawText method to Visual3d_Layer class. Fixes issue #30
Merge pull request #34 from tpaviot/review/Visual3d-fix

Added missing DrawText method to Visual3d_Layer class. Fixes issue #30
Changed the default material
Added size; fixed ipython --gui='qt'
Merge pull request #31 from tpaviot/review/gui-improvements

Review/gui improvements
PEP8 improvements; removed menu item for atomic examples ; misc fixes/improvements
Added core_simple_mesh example
Merge pull request #32 from tpaviot/review/more-examples

Review/more examples