Support for MySQL's MyISAM and other non-InnoDB engines

Don't generate foreign keys for MySQL tables that are not InnoDB.

This PR adds a `supportsForeignKeyConstraintBetween($table1, $table2)` method to Platforms allowing to check if foreign keys between two tables are supported (e.g. on MySQL: InnoDB to InnoDB FK is OK, Memory to InnoFB is not).

The MySQL implementation is also provided and `CreateSchemaSqlCollector` has been adapted to use this feature. 

This PR is related to https://github.com/doctrine/doctrine2/pull/865