getting sqlalchemy datetime type ignores None timezones
When converting to datetime64, pandas does:

``` python
        if col_type == 'datetime64': 
            try:
                tz = col.tzinfo
                return DateTime(timezone=True)
            except:
                return DateTime
        if col_type == 'timedelta64':
```

2 problems
1. The exception catch is way to broad. I think this was probably supposed to catch an AttributeException (For col with no tzinfo attribute at all)
2. columns can have a tzinfo =None, in which case a Datetime(timezone=False) should be created instead.
