calls a method called 'allow' in our options that will
perform any access checking

@author Tim Golen 2015-03-01
@access private

@param  {string} action the CRUD actions being done
@param  {object} req an Express or Restify Request object
@param  {object} res an Express or Restify Response object
@param  {Function} cb
if request is allowed, cb(null, true)
if request is not allowed, cb(null, false)

@return {void}
function allow(action, req, res, cb) {
		// call the allow method in our options if it exists
		// or we will just assume it is always allowed
		if (options.allow) {
			return options.allow.apply(this, arguments);
		}

		return cb && cb(null, true);
	}