BUG: apply Timestamp to resultant datetimes when operating on offsets

@filmor

I pushed up a commit to your branch https://github.com/jreback/pandas/commit/38e4d4660912c9132d2baa5f3fe1fc0d84e45e33

add this to this PR, should fix the issue....its a 'bug' sort of in the return type of adding an offset to a timestamp with a tz

as an aside this exposes a bug in your PR.