implemented database functions to create/destroy friendship
added friendship application methods
added friendship test functions
handling 'protected' parameter when updating/creating/reading user details
implementation of Application::get_user_detais_secured() method to get user information depending on 'protected' status & friendship
updated test suite to test Application::get_user_details_secured() method
added validator function for tags
additional username parameter in Application::get_tags() methods
code review