Use absolute import for OCCViewer; fix an inheritance issue

This fixes issue #25
importing a module ( file ) from the same directory is only possible with the relative import syntax
however, I prefer the more consistent "from OCC.Display import X" syntax

finally, `apply` is no longer a keyword in py3k, so that's updated to unpacking the arguments form a tuple
PEP8 improvements; removed menu item for atomic examples ; misc fixes/improvements
Added core_simple_mesh example