DateField should parse on attribution

As esengine uses dateutils.parser it can handle date parsing on attribution.

> only when _strict == False

``` python
class Doc(Document):
    date = DateField()

doc = Doc()

doc.date = "2010-01-01"
doc.date = "2010-01-01 15:30:22"
doc.date = "15/03/2010"

# should all be parsed to datetime.datetime(2010, 01, 01)

isinstance(doc.date, datetime)
True
```
