Update foundation.magellan.js

- passed method parameter of init method to its call of set_threshold
- added if condition to set_threshold method to make it capable to react to the new given parameter (options)
- in set_active_class_method: renamed options parameters attribute from active_class to active

now the module behaves exactly as it is proposed on the project's website (http://foundation.zurb.com/docs/components/magellan.html)