implementing issue 28. commit before test on dev1 server. (12)

28) add share icons to api_gift (one icon for each api provider)
    OK for facebook, google+, linkedin, Pinterest, twitter and vkontakte.
    todo: add ajax spinner while fetching share url from server. Normally quick, but server is slow and small.
    todo: change error handling in get_share_gift_link. Do not use alert. Use ajax error table under gift links
    todo: check http://developer.sharethis.com/Web_Share_API
    todo: remove max_lng parameter from share_gift(provider, gift_id, link, max_lng, extra)
    todo: add try catch blocks in share_gift* methods to handle JS exceptions