Add FreeMarker support

This commit adds auto-configuration and a starter,
spring-boot-starter-freemarker, for using FreeMarker view templates in
a web application.

A new abstraction, TemplateAvailabilityProvider, has been introduced.
This decouples ErrorMvcAutoConfiguration from the various view
technologies that Spring Boot now supports, allowing it to determine
when a custom error template is provided without knowing the details of
each view technology.

Closes #679

spring-boot-samples/spring-boot-sample-web-freemarker/src/main/java/sample/freemarker/SampleWebFreeMarkerApplication.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2012-2014 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package sample.freemarker;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.context.annotation.ComponentScan;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration
+@EnableAutoConfiguration
+@ComponentScan
+public class SampleWebFreeMarkerApplication {
+
+	public static void main(String[] args) throws Exception {
+		SpringApplication.run(SampleWebFreeMarkerApplication.class, args);
+	}
+
+}

spring-boot-samples/spring-boot-sample-web-freemarker/src/main/java/sample/freemarker/WelcomeController.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2012-2014 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package sample.freemarker;
+
+import java.util.Date;
+import java.util.Map;
+
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.RequestMapping;
+
+@Controller
+public class WelcomeController {
+
+	@Value("${application.message:Hello World}")
+	private String message = "Hello World";
+
+	@RequestMapping("/")
+	public String welcome(Map<String, Object> model) {
+		model.put("time", new Date());
+		model.put("message", this.message);
+		return "welcome";
+	}
+
+}