converted plugin code to utf-8

TandemPlugin.class.php

assets/css/TandemPlugin.css

assets/javascript/TandemPlugin.js

classes/TandemManager.class.php

controllers/admin.php

controllers/my_tandems.php

controllers/pair.php

controllers/profile.php

migrations/02_add_data.php

plugin.manifest

templates/start.php

views/_common/_profile.php

views/admin/config.php

views/admin/search.php

views/admin/terminated_pairs.php

views/my_tandems/index.php

views/my_tandems/manage_mother_languages.php

views/my_tandems/pairs.php

views/my_tandems/profiles.php

views/pair/decide.php