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.