Python iterators of dicts

The Python backend now supports sequences of dictionaries as well as sequences of tuples.  This is particularly valuable when dealing with JSON files.

```Python
In [1]: from blaze import *

In [2]: d = [{'name': 'Alice', 'amount': 100, 'id': 1},
   ...:      {'name': 'Bob', 'amount': 200, 'id': 2},
   ...:      {'name': 'Alice', 'amount': 50, 'id': 3}]

In [3]: t = Symbol('t', 'var * {name: string, amount: int, id: int}')

In [4]: compute(2*t.amount, d)
Out[4]: <itertools.imap at 0x7f98b6b851d0>

In [5]: list(_)
Out[5]: [200, 400, 100]
```

This also actually uses `pre_compute` which I now define to be a function that operates on each leaf of data to a computation.

```Python
def pre_compute(expr, data):
    return some-transformation-on-data
```