major changes:

- The self._grad_op now only returns zeros_like() for special types like
    NullType() or DisconnectedType()
- call to grad() will furthur replace returned zero tensors with special
    types
- proposed gradient override interface :
    (single or list of below)
    Ellipsis -> <no_override>
        (-) since python 2 does not support `[...]` syntax, this may
        result in uglier code in python 2
    None -> NullType()
    int(0) -> DisconnectedType()
    OpFromGraph instance or callable -> <override>

minor changes:

- various typo/bug fix

notes:

- This commit breaks OpFromGraph.R_op, which is expected to be fixed in
upcoming commits.