- Added "unicode_errors" parameter to String, Unicode, etc.

- Added "unicode_errors" parameter to String, Unicode, etc.
Behaves like the 'errors' keyword argument to
the standard library's string.decode() functions.   This flag
requires that `convert_unicode` is set to `"force"` - otherwise,
SQLAlchemy is not guaranteed to handle the task of unicode
conversion.   Note that this flag adds significant performance
overhead to row-fetching operations for backends that already
return unicode objects natively (which most DBAPIs do).  This
flag should only be used as an absolute last resort for reading
strings from a column with varied or corrupted encodings,
which only applies to databases that accept invalid encodings
in the first place (i.e. MySQL. *not* PG, Sqlite, etc.)