support field comments in generated class files.

Provide a SimpleCommentGenerator, to add comment for java class fields.

If you add comments for your columns when creating a table, the comments will be added to the fields in java class files.

For example, the following column in table:

`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT 'auto incremented id',

and the generated fields:

     /**
     * auto incremented id
     */
    private Long id;