Test compute numpy

This is almost ready for merging. I have one problem though, with the function on line 49:

```
@dispatch(BinOp, base, np.ndarray)
def compute_up(t, lhs, rhs, **kwargs):
    return t.op(lhs, rhs)
```

Had a lot of trouble figuring out how to call this function, figuring out what it wanted for inputs. Any feedback on this would be appreciated. 
So i guess a general theme of my recent comments is that testing code should be clear to the reader, just like normal code.  Actually, it may have even stricter requirements because readers will encounter this test when something goes wrong and they'll want to understand the context given just this small piece of code.