Add support for fallback factories in feign client annotation. (#1373)

Adds `@FeignClient.fallbackFactory` to define a `feign.hystrix.FallbackFactory`.

fixes gh-1117

docs/src/main/asciidoc/spring-cloud-netflix.adoc

@@ -1012,6 +1012,30 @@ static class HystrixClientFallback implements HystrixClient {
 }
 ----
 
+If one needs access to the cause that made the fallback trigger, one can use the `fallbackFactory` attribute inside `@FeignClient`.
+
+[source,java,indent=0]
+----
+@FeignClient(name = "hello", fallbackFactory = HystrixClientFallbackFactory.class)
+protected interface HystrixClient {
+	@RequestMapping(method = RequestMethod.GET, value = "/hello")
+	Hello iFailSometimes();
+}
+
+@Component
+static class HystrixClientFallbackFactory implements FallbackFactory<HystrixClient> {
+	@Override
+	public HystrixClient create(Throwable cause) {
+		return new HystrixClientWithFallBackFactory() {
+			@Override
+			public Hello iFailSometimes() {
+				return new Hello("fallback; reason was: " + cause.getMessage());
+			}
+		};
+	}
+}
+----
+
 WARNING: There is a limitation with the implementation of fallbacks in Feign and how Hystrix fallbacks work. Fallbacks are currently not supported for methods that return `com.netflix.hystrix.HystrixCommand` and `rx.Observable`.
 
 [[spring-cloud-feign-inheritance]]