Dynd

A very simple dynd backend

```Python
In [1]: from blaze import *

In [2]: s = Symbol('s', '3 * var * {name: string, amount: int}')

In [3]: x = nd.array([[('Alice', 1), ('Bob', 2)],
                 [('Charlie', 3)],
                 [('Dennis', 4), ('Edith', 5), ('Frank', 6)]],
                type = str(s.dshape))

In [4]: compute(s.amount * 100, x)
Out[4]: 
nd.array([     [100, 200],           [300], [400, 500, 600]],
         type="strided * var * int32")
```

Note that in this case, Blaze is kind of a no-op.  The syntax that both support is identical.

This is building off of a couple of array PRs.  This should clean up as those are merged.  

cc @mwiebe