The ``op.drop_table()`` operation directive will now trigger the
``before_drop()`` and ``after_drop()`` DDL event hooks at the table level,
which is similar to how the ``before_create()`` and ``after_create()``
hooks are triggered by the ``op.create_table()`` directive. Note that as
``op.drop_table()`` accepts only a table name and optional schema name, the
``Table`` object received by the event will not have any information within
it other than the table name and schema name.

Fixes: #1037
Closes: #1036
Pull-request: #1036
Pull-request-sha: ea44e7f

Change-Id: I20a1702e17ed88054206d964152ce05b81d0f89e