Refined Spring Social Auto-Configuration

Many changes in this PR:

 * Only auto-configure FB/Twitter/LinkedIn if the corresponding app-id and app-secret properties are set.
 * Only configure ConnectController and ProviderSignInController if there is a ConnectionFactoryLocator.
 * Auto-configure Spring Social's SpringSocialDialect for Thymeleaf if Thymeleaf is present.
 * Added several tests around Spring Social auto-configuration.

In addition to the automated tests mentioned above, I also manually tested the changes via the Spring Boot-enabled Spring Social Showcase at https://github.com/spring-projects/spring-social-samples/tree/master/spring-social-showcase-boot. 

I also manually tested the changes against some CLI-driven example code I have written at https://github.com/habuma/spring-boot-examples/tree/master/social.