Add JdbcOperationsSessionRepository

This commit provides implementation of SessionRepository based
on Spring's JdbcOperations interface.

@EnableJdbcHttpSession annotation is provided to ease the
configuration, together with spring-session-jdbc BOM and schema
creation scripts for all major databases.

Fixes gh-364

samples/httpsession-jdbc/src/main/java/sample/Initializer.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2014-2016 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;
+
+import org.springframework.session.web.context.AbstractHttpSessionApplicationInitializer;
+
+// tag::class[]
+public class Initializer
+		extends AbstractHttpSessionApplicationInitializer { // <1>
+
+	public Initializer() {
+		super(Config.class); // <2>
+	}
+}
+// end::class[]