explicit engine encoding for unicode

[Former user]

String and Unicode types are capable of performing automatic str/unicode conversions, but allways use utf-8 encoding. It would be wery usefull to be able to specify encoding other than utf-8.

[Former user]

The proposed patch handles encodings on engine level, while some databases (for example MySQL) support different encodings for different tables inside a database (and maybe even different encodings for each column inside the same table).

I guess, the type itself (String/Unicode) should be extended to support specific encoding, and if not specified to use the engine's encoding.

[Mike Bayer]

reasonable enough, this is committed in changeset:1125