Add ability to use a css class on fieldsets

Using fieldsets changes from:

    fieldsets: [
        {legend: 'fieldset 1', fields: ['name', 'othername']},
        {legend: 'fieldset 2', fields: ['password', 'password-repeat']}
    ]

to 

    fieldsets: [
        {legend: 'fieldset 1', fields: ['name', 'othername'], cssClass: 'names'},
        {legend: 'fieldset 2', fields: ['password', 'password-repeat'], cssClass: 'passwords'}
    ]