Improve creating_an_op.rst

Changes:
1. Remove references to c-code which apply to `COp` but not `Op`
2. Improve explanation of `make_node`
3. Emphasize distinction between itypes/otypes and make-node
4. Show how to test `grad` and `infer_shape` implementation
5. Introduce more complex multi-output Op to drive these details home

Author: ricardoV94