Boot activiti

Hi, 

Now that Spring Boot is GA, I want to include a Boot auto-configuration integration. With this, someone needs only link to the Activiti Spring Boot module (which this PR contributes) and - assuming they have a valid `javax.sql.DataSource` defined in the context and they've deployed process definitions in the `src/main/resources/process` directory - the user need only inject a pointer to the fully configured `ProcessEngine` instance, like this:

```
@Autowired 
 public void setProcessEngine(ProcessEngine pe) { 
 this.processEngine = pe; 
}```

Done!