Support for MySQL utf8mb4

As outlined in http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-upgrading.html

> InnoDB has a maximum index length of 767 bytes, so for utf8 or utf8mb4 columns, you can index a maximum of 255 or 191 characters, respectively. If you currently have utf8 columns with indexes longer than 191 characters, you will need to index a smaller number of characters. In an InnoDB table, these column and index definitions are legal:

This change ensures those using the InnoDB storage engine in MySQL with utf8mb4_* collations can use reversion.