fixes issues ensure_lookup_table and grease_pencil, related to new blender 2.73 behavior.

#    blender 2.73 needs to call ensure_lookup_table() for bm.verts[], bm.edges[], bm.faces[].
#    generically the fix will do this...
#    the lookup_table will get "dirty" after:
#    bm.new(), bm.from_mesh(), bm.from_edit_mesh()
#    bm.verts.new(), bm.edges.new(), bm.faces.new()
#    bm.verts.remove(), bm.edges.remove(), bm.faces.remove()
#    bm.normal_update(), bm.copy()
#
#    bm.verts.ensure_lookup_table() ### 2.73
#    bm.edges.ensure_lookup_table() ### 2.73
#    bm.faces.ensure_lookup_table() ### 2.73

#    blender 2.73 has a new grease_pencil per object and new per scene
#    gp = object.grease_pencil
#    if not gp:
#        gp = context.scene.grease_pencil