default Unicode to use UNICODE charset in mysql

[Mike Bayer]

see https://github.com/zzzeek/sqlalchemy/pull/102

[Mike Bayer]

we've decided we're not comfortable hardwiring the encoding of the column like this. While some DBs require NVARCHAR for unicode, w/ MySQL there's a lot of options for a plain VARCHAR and many of them only need the backend to have the right encoding.