- Fixed a regression 0.8 whereby the "multidb" environment template

- Fixed a regression 0.8 whereby the "multidb" environment template
failed to produce independent migration script segments for the
output template.  This was due to the reorganization of the script
rendering system for 0.8.  To accommodate this change, the
:class:`.MigrationScript` structure will in the case of multiple
calls to :meth:`.MigrationContext.run_migrations` produce lists
for the :attr:`.MigrationScript.upgrade_ops` and
:attr:`.MigrationScript.downgrade_ops` attributes; each :class:`.UpgradeOps`
and :class:`.DowngradeOps` instance keeps track of its own
``upgrade_token`` and ``downgrade_token``, and each are rendered
individually.
fixes #318