2.6 Add Support for MySQL Table Comments

Adds suppport for reading and writing table comments.

Example:
```` php
public $backups = array(
		'id' => array('type' => 'integer', 'null' => false, 'default' => null, 'unsigned' => false, 'key' => 'primary'),
		// Some fields
		'created' => array('type' => 'datetime', 'null' => true, 'default' => null),
		'created_by' => array('type' => 'integer', 'null' => true, 'default' => null, 'unsigned' => false),
		'modified' => array('type' => 'datetime', 'null' => true, 'default' => null),
		'modified_by' => array('type' => 'integer', 'null' => true, 'default' => null, 'unsigned' => false),
		'indexes' => array(
			'PRIMARY' => array('column' => 'id', 'unique' => 1)
		),
		// Table comment gets added at the end of tableParameters
		'tableParameters' => array('charset' => 'utf8', 'collate' => 'utf8_general_ci', 'engine' => 'InnoDB', 'comment' => 'foo bar baz1')
	);
````

Creating the table:
`````sql
CREATE TABLE `cakephp_test`.`backups` (
        `id` int(11) NOT NULL AUTO_INCREMENT,
       /* some fields */
        `created` datetime DEFAULT NULL,
        `created_by` int(11) DEFAULT NULL,
        `modified` datetime DEFAULT NULL,
        `modified_by` int(11) DEFAULT NULL,     PRIMARY KEY  (`id`))    DEFAULT CHARSET=utf8,
        COLLATE=utf8_general_ci,
        ENGINE=InnoDB,
        COMMENT='foo bar baz1';
````

Altering the table:
```` sql
ALTER TABLE `cakephp_test`.`backups`
        COMMENT='foo bar baz';
````

Altering the table when comment becomes empty string:
```` sql
ALTER TABLE `cakephp_test`.`backups`
        COMMENT='';
````