Preserve comment and info in Drop/CreateTableOp

Preserve comment and info in Drop/CreateTableOp

Fixed regression due to :ticket:`803` where the ``.info`` and ``.comment``
attributes of ``Table`` would be lost inside of the :class:`.DropTableOp`
class, which when "reversed" into a :class:`.CreateTableOp` would then have
lost these elements. Pull request courtesy Nicolas CANIART.

Fixes: #879
Closes: #881
Pull-request: https://github.com/sqlalchemy/alembic/pull/881
Pull-request-sha: e509b9e8180085998e32746721eeb9266b9f2145

Change-Id: I84a0680266fcf80f3dc922f3cbc76dabd74eedd3