- added new flag to String and create_engine(), assert_unicode=(True|False|None).

- added new flag to String and create_engine(), assert_unicode=(True|False|None).
When convert_unicode=True, this flag also defaults to `True`, and results in all
unicode conversion operations raising an exception when a non-unicode bytestring
is passed as a bind parameter.  It is strongly advised that all unicode-aware
applications make proper use of Python unicode objects (i.e. u'hello' and
not 'hello').