Scalar coercions column

Scalar coercion on optional dtypes

```Python
>>> from blaze import *
>>> x = ScalarSymbol('x', '?date')
>>> x > '2012-01-01'
x > 2012-01-01
>>> (x > '2012-01-01').rhs
datetime.date(2012, 1, 1)
```

Fixed an issue where scalar coercions didn't propagate over to columns

```Python
>>> t = TableSymbol('t', '{name: string, amount: int, timestamp: ?date}')
>>> (t.timestamp < '2014-12-01').expr.rhs
date(2014, 12, 1)
```