Added activity destinations endpoint

Added new endpoint to get all destinations from an activity.
examples:

/api/v3/activities/0776667e-e581-44ea-b758-894173c0bd28/destinations
[
    {
        "name": "Cancún",
        "code": "cancun",
        "content": "This Mexican paradise on the Yucatan Peninsula is perfect for the traveler who likes to combine beautiful nature with fascinating culture. The Caribbean is right there inviting you to swim, snorkel and go fishing. For some real beach-holiday relaxation, take a boat trip out to fabulous Isla Mujeres, where you can snorkel off the reef and then enjoy the beach club's facilities. Cocktails are optional. Cancun is also, of course, about the Mayan civilization. A trip to the vast pyramidal temple complex of Chichen Itza is an experience best had in the early morning before the heat gets too oppressive. If history is your thing, you'll also need to visit the archeological zone of Coba, whose Nohoch Muul pyramid is the tallest in the Yucatan. At the walled Mayan city of Tulum, you'll be charmed by the ruins right alongside the sapphire waters. When you've had enough pyramids and soothing facials, go crazy with activities such as cliff diving, rope swinging, cycling, kayaking and rope-bridge walking at Xel-Ha Eco-waterpark, which also has a truly impressive natural aquarium. Remember to bring your waterproof sunscreen!",
        "meta_description": "Musement provides the best handpicked tours and activities in Cancún. Skip the lines to world famous sites and museums, and make your trip unforgettable. Buy online now!",
        "latitude": 21.163,
        "longitude": -86.85,
        "cover_image_url": "http://images.musement.com/cover/0001/43/cancun_header-42360.jpeg(294 kB)
http://images.musement.com/cover/0001/43/cancun_header-42360.jpeg
",
        "time_zone": "America/Cancun",
        "default": false
    },
    {
        "name": "Riviera Maya-Playa del Carmen",
        "code": "riviera maya-playa del carmen",
        "content": "La Riviera Maya es un destino turístico conocido en el mundo entero por sus cenotes de aguas azules, sus espectaculares playas de arena blanca y sus impresionantes ruinas mayas. La Riviera Maya se extiende a lo largo de unos 135 km de litoral, desde Puerto Morelos hasta Tulum, y ofrece algo para todos los gustos. Elevadas sobre las playas se encuentran las antiguas ruinas mayas, que sumergen al visitante en el pasado, mientras que la cercana localidad de Playa del Carmen acoge a turistas de todas partes en busca de fiesta. A pesar de que la región alberga numerosos complejos de tipo todo incluido, también es posible escapar de los habituales circuitos turísticos y visitar pequeñas localidades de pescadores y playas casi vírgenes por todo el litoral. Con su amplia oferta de actividades y destinos culturales, ¡ten por seguro que en la Riviera Maya podrás adaptar tu viaje a todos tus intereses y necesidades!",
        "latitude": 20.633282,
        "longitude": -87.086489,
        "cover_image_url": "http://images.musement.com/cover/0003/14/fotolia-164681864-subscription-xl-jpg_header-213580.jpeg(2 MB)
http://images.musement.com/cover/0003/14/fotolia-164681864-subscription-xl-jpg_header-213580.jpeg
",
        "time_zone": "America/Mexico_City",
        "default": true
    }
]