ENH: add Series.astype with the new tz dtype

[jreback]

add ability to astype using the new dtype

[jorisvandenbossche]

@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

[jreback]

@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).

[jreback]

and we allow s_aware.astype('datetime64[ns]') so this is just the other direction