Add keepdims kwarg to Reduction

```Python
In [1]: from blaze import *

In [2]: x = Symbol('x', '5 * 3 * int')

In [3]: x.sum(axis=0, keepdims=True)
Out[3]: sum(_child=x, axis=(0,), keepdims=True)

In [4]: x.sum(axis=0, keepdims=True).dshape
Out[4]: dshape("1 * 3 * int32")

In [5]: x.sum(axis=0).dshape
Out[5]: dshape("3 * int32")
```