`Angles at edges' node

Created by: portnov

Given a mesh, calculate angle at each edge. See http://www.blender.org/api/blender_python_api_2_72_1/bmesh.types.html#bmesh.types.BMEdge.calc_face_angle This partially corresponds to https://github.com/nortikin/sverchok/blob/obj_sockets/nodes/analyzer/bmesh_verts.py