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. 
If we're not testing this kind of expression then we should go ahead and test it.  It might be better to ensure that we get the exact result though.  E.g. 

```Python
expr = t[t['amount'] < 0]['name']
result = compute(expr, y)
assert (result == np.array(['Bob', 'Edith'])).all()
```

I am surprised that we weren't testing projections and selections though.  Perhaps these can be added to the style of the tests above?