Add Spring Social autoconfiguration

Basic autoconfig, starter, and CLI support for Spring Social Facebook, Twitter, and LinkedIn. 

Assuming that the application's credentials are set (as in application.properties or otherwise), the following is a runnable Facebook application via the CLI:

```java
@Controller class H{Facebook f;@RequestMapping("/")String h(){"redirect:/connect/facebook"}}
```

The properties that must be set are spring.social.facebook.appId and spring.social.facebook.appSecret. 

Similarly, the following are complete Twitter and LinkedIn examples:

.Twitter Sample (requires spring.social.twitter.appId and spring.social.twitter.appSecret properties)
```java
@Controller class H{Twitter t;@RequestMapping("/")String h(){"redirect:/connect/twitter"}}
```

.LinkedIn Sample (requires spring.social.linkedin.appId and spring.social.linkedin.appSecret properties)
```java
@Controller class H{LinkedIn l;@RequestMapping("/")String h(){"redirect:/connect/linkedin"}}
```

This does not (yet) support signin via a provider, but that is among the next items I intend to implement. I just wanted to go ahead and submit this initial set of work first before moving on to any additional functionality.