Support any unicode charsets for MySQL.

MySQL unicode support is not only `utf8mb4`.
Then, The index length problem is not only `utf8mb4`.

http://dev.mysql.com/doc/refman/5.6/en/charset-unicode.html

    SELECT * FROM information_schema.character_sets WHERE maxlen > 3;
    +--------------------+----------------------+------------------+--------+
    | CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION      | MAXLEN |
    +--------------------+----------------------+------------------+--------+
    | utf8mb4            | utf8mb4_general_ci   | UTF-8 Unicode    |      4 |
    | utf16              | utf16_general_ci     | UTF-16 Unicode   |      4 |
    | utf16le            | utf16le_general_ci   | UTF-16LE Unicode |      4 |
    | utf32              | utf32_general_ci     | UTF-32 Unicode   |      4 |
    +--------------------+----------------------+------------------+--------+

In addition, the charset of `version` column in `schema_migrations`
table is depend on the database default charset and collation
rather than encoding of connection.