Prettier printing of Expr instances

Before:

```
In [1]: from blaze import TableSymbol

In [2]: t = TableSymbol('t', '{id: int, amount: float64, name: string}')

In [3]: from datetime import datetime

In [4]: t.amount.map(datetime.fromtimestamp)
Out[4]: Map(t['amount'], <built-in method fromtimestamp of type object at 0x1014cb4c0>, None, None)
```

After

```
In [1]: from blaze import TableSymbol

In [2]: t = TableSymbol('t', '{id: int, amount: float64, name: string}')

In [3]: from datetime import datetime

In [4]: t.amount.map(datetime.fromtimestamp)
Out[4]: Map(child=t['amount'], func=datetime.fromtimestamp, _schema=None, _iscolumn=None)
```