Fix for issue #456. 

Fix for issue #456. Flyway is unable to correctly find the quote delimiter when there is an inline charset cast on the string literal. I added a new method to the `SqlStatementBuilder.java` that can be overridden in the dialect specific subclasses if other dialects need similar behavior. For the `MySQLStatementBuilder.java`, I added a list of the supported charsets and check to make sure the tokens do not contain a charset cast prefix. If so, then I remove the cast reference so the token can be evaluated properly for quotes.