Scope validation for service

Hi!
After this changes https://github.com/Lusitanian/PHPoAuthLib/commit/8a54c0d5bfb42cd6f055101c5155a574f0a2642c#diff-dee79ba6fbfaaad0aa5386d490e1ac7f ServiceProxy does not work. 
I'm extend method for scope validation for vallidating service.
Init method has been removed, service and state initialization moved to consructor, because scope validation running in construction method (see https://github.com/Lusitanian/PHPoAuthLib/blob/master/src/OAuth/OAuth2/Service/AbstractService.php#L47) 

Thanks for attention.