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-xml/src/main/java/sample/SessionServlet.java

@@ -0,0 +1,36 @@
+/*
+ * 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 javax.servlet.*;
+import javax.servlet.http.*;
+import java.io.IOException;
+
+// tag::class[]
+public class SessionServlet extends HttpServlet {
+
+	@Override
+	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
+		String attributeName = req.getParameter("attributeName");
+		String attributeValue = req.getParameter("attributeValue");
+		req.getSession().setAttribute(attributeName, attributeValue);
+		resp.sendRedirect(req.getContextPath() + "/");
+	}
+
+	private static final long serialVersionUID = 2878267318695777395L;
+}
+
+// end::class[]