compute: add numpy backend

Mostly I copied the Pandas test suite but swapped out DataFrames for structured arrays.  This is eventually for PyTables interactions.

```Python
In [1]: from blaze import *

In [2]: L = [(1, 'Alice',   100), (2, 'Bob', -200), (3, 'Charlie', 300),
(4, 'Denis',   400), (5, 'Edith',  -500)]

In [3]: x = np.array(L, dtype=[('id', int), ('name', 'O'), ('amount', int)])

In [4]: t = Table(x)

In [5]: t
Out[5]: 
   id     name  amount
0   1    Alice     100
1   2      Bob    -200
2   3  Charlie     300
3   4    Denis     400
4   5    Edith    -500

In [6]: t.amount[t.name == 'Alice']
Out[6]: 
     0
0  100

In [7]: t.amount.sum()
Out[7]: '100'
```