FIX sql handling of timedelta64 columns (GH6921)
Closes #6921

Converting to an int was actually also how it was now with sqlite DBAPI fallback (as timedelta64 is subclass of integer), so this solution is also backwards compatible and consistent between sqlalchemy and fallback mode.

@danielballan @jreback 
 don't you need to explicity cast before you write to the db itself? (e.g. doing `s.astype('int64')`, essentially converting it to an integer column)
 Not really necessary, as it is an int subclass, and in the sql write code `np.asscalar` is used to convert the numpy scalars to python types:

```
In [29]: np.asscalar(np.timedelta64(10000, 'ns'))
Out[29]: 10000L
```
 Ah, and it seems I have to skip the tests for older numpy versions. Updated.
 looks good
 def put this as a warning in the docs though
 OK, added a note in the docs. 
