ENH: add Series.astype with the new tz dtype
add ability to astype using the new dtype

@jreback I am not really sure about this. For me, it is not very clear if this would be doing a tz_convert or a tz_localize. 

And since we already have `tz_convert` and  `tz_localize`, I don't really see the added value of advertising the use of `astype('datetime64[ns, tz=...]')` as an alternative
 @jorisvandenbossche is just for compat really. I think people would try to do it. its actually very natural once you try it! (think about tz as part of the dtype, then this makes perfect sense).
 and we allow `s_aware.astype('datetime64[ns]')` so this is just the other direction
