additional methods for SecurityService

add getCurrentUser method to SecurityService for getting current user from spring security context and retrieve User object that applies to Authentication object(from s.s. context)
add getCurrentUserUsername method to SecurityService for getting current user username
add authenticateUser method to SecurityService for logging in user after registration and may be needed for future functional such as change password
add registereUser method to UserService with check for user existance, create User and save to DB from passed data