Add @IntegrationTest annotation for test classes

Allowing use of embededded container in @SpringApplicationConfiguration
tests, e.g.

```
@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = Config.class)
@WebAppConfiguration
@IntegrationTest
public class SpringApplicationIntegrationTestTests {

	@Test
	public void nestedConfigClasses() {
		String body = new RestTemplate().getForObject("http://localhost:8080/",
				String.class);
		assertEquals("Hello World", body);
	}

	@Configuration
	@EnableWebMvc
	@RestController
	protected static class Config {

		@Bean
		public DispatcherServlet dispatcherServlet() {
			return new DispatcherServlet();
		}

		@Bean
		public EmbeddedServletContainerFactory embeddedServletContainer() {
			return new TomcatEmbeddedServletContainerFactory();
		}

		@RequestMapping("/")
		public String home() {
			return "Hello World";
		}

	}

}
```

Fixes gh-473