SOCIAL-255: Added ProviderSignInInterceptor and calls to interceptors during ProviderSignInController's sign-in flow. Does not address cases where PSIC must redirect to application-defined signup process.
Merge pull request #84 from habuma/master

SOCIAL-255: Added ProviderSignInInterceptor and calls to interceptors during ProviderSignInController's sign-in flow. Does not address cases where PSIC must redirect to application-defined signup process.