Validates that the given value is present in the configured `list`.
For example:

validations: [{type: 'inclusion', field: 'gender', list: ['Male', 'Female']}]

@param {Object} config Config object
@param {String} value The value to validate
@return {Boolean} True if the value is present in the list
function(config, value) {
        return config.list && Ext.Array.indexOf(config.list,value) != -1;
    }