support for Map of Columnwise

Needed to support map of columnwise:

```
data = [(1.0, 2.0, 'USA'),
        (1.0, 3.1, 'RUSSIA'),
        (2.2, 4.0, 'USA'),
        (1.0, 2.8, 'RUSSIA')]


s = '{t1:float64, t2:float64, name: string}'
t = TableSymbol('test', s)

tfinal = (t['t2']-t['t1']).map(lambda x: int(x), schema="{diff: int32}", iscolumn=True )
```

@mrocklin, interested in your thoughts. Needs additional changes to be more general.