TypeError in tangent.grad for array method.

[John Kitchin]

This may be related to issue #18

def f(x):
    return x.sum()

df = tangent.grad(f)

but it raises a different kind of error:

TypeError: unhashable type: 'numpy.ndarray'

This works fine:

def f(x):
    return np.sum(x)

df = tangent.grad(f)
print df(np.array([1, 2, 3]))

#+RESULTS:
:RESULTS:
[1 1 1]

[Dan Moldovan]

Duplicate of #18