[OAuth] initial loginOAuthProvider flow

[carmenlau]

modified #98 #99

• -[SKYContainer.auth loginOAuthProvider:(NSString*)providerID, options:(NSDictionary*)options completion:(void(^)(NSError*, SKYUser*))]
  • Create or login a new skygear user, associated with the provider
  • providerID - A string that identify the login provider
    • We will provide com.facebook, com.google
  • options
    • scope
    • scheme - For redirect after authorization flow
  • This function returns a skygear user, and an access token of the service,
    via a delegate.