Truncation

For binning operations we want truncation operations.  This is important both for arithmetic truncation

```Python
>>> from blaze import Symbol, compute
>>> x = Symbol('x', 'real')
>>> compute(x.truncate(10), 123)
120
>>> compute(x.truncate(0.1), 3.1415)
3.1
```

And for datetime truncation (in progress)

Datetime truncation seems to be pretty doable in Pandas / NumPy. Python is a bit more of a pain.  No clue on SQLAlchemy.  Presumably we'd have to go into each of the databases independently.