Add support for specifying PostgreSQL index storage paramters (e.g.
fillfactor).

:func:`.engine_from_config` were not being parsed correctly;
these included ``pool_threadlocal`` and the psycopg2 argument
``use_native_unicode``. fixes #3435
- add legacy_schema_aliasing config parsing for mssql
- move use_native_unicode config arg to the psycopg2 dialect

with the psycopg2 dialect in conjunction with non-ascii values
and ``native_enum=False`` would fail to decode return results properly.
This stemmed from when the PG :class:`.postgresql.ENUM` type used
to be a standalone type without a "non native" option.
fixes #3354
- corrected the assertsql comparison rule to expect a non-ascii
SQL string

:ticket:`3184` would cause index operations to fail on Postgresql
versions 8.4 and earlier.  The enhancements are now
disabled when using an older version of Postgresql.
fixes #3343

DROP TYPE instruction when a plain ``table.drop()`` is called,
assuming the object is not associated directly with a
:class:`.MetaData` object.   In order to accomodate the use case of
an enumerated type shared between multiple tables, the type should
be associated directly with the :class:`.MetaData` object; in this
case the type will only be created at the metadata level, or if
created directly.  The rules for create/drop of
Postgresql enumerated types have been highly reworked in general.
fixes #3319