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. 
E.g. perhaps we add to these tests.  I notice that the first test here, `test_projection` should be renamed now to `test_field`:

```Python
def test_projection():
    assert eq(compute(t['name'], x), x['name'])

def test_selection():
    assert eq(compute(t[t['amount'] == 100], x), x[x['amount'] == 0])
    assert eq(compute(t[t['amount'] < 0], x), x[x['amount'] < 0])
```

So maybe something like 

```Python
def test_field():
    assert eq(compute(t['name'], x), x['name'])

def test_projection():
    assert eq(compute(t[['name', 'amount']], x), x[['name', 'amount']])
```