Datetime should be mapped to timesamp "with time zone" instead of "without time zone"

Hi, there is currently no way to create a column with the type `timestamp with time zone`.

When I look into the code I found that `datetime` is mapped to `timestamp`. So no matter if I define a `timestamp` or a `datetime` I will have a `timezone`.

In the [MySQL documentation for `DATETIME`](http://dev.mysql.com/doc/refman/5.7/en/datetime.html) you can see that this type of column takes care of local time zone.

Is there something wrong to map `datetime` to `timestamp with time zone` ?

I tried to update the mapping locally and all work fine for me. Do you see any issue doing this ?

If not I can submit a PR with these changes.

Thanks.
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