Add GemFire Support

Fixes GH PR #148 and PR #308 implementing a GemFire Adapter to support
clustered HttpSessions in Spring Session.

* Resolve SGF-373 - Implement a Spring Session Adapter for GemFire backing
a HttpSession similar to the Redis support.
* Add Spring Session annotation to enable GemFire support with
@EnableGemFireHttpSession.
* Add extesion of SpringHttpSessionConfiguration to configure GemFire using
GemFireHttpSessionConfiguration.
* Add implementation of SessionRepository to access clustered, replicated
HttpSession state in GemFire with GemFireOperationsSessionRepository.
* Utilize GemFire Data Serialization framework to both replicate
HttpSession state information as well as handle deltas.
* Utilize GemFire OQL query to lookup arbitrary Session attributes by name,
and in particular the user authenticated principal name.
* Implment unit and integration tests, and in particular, tests for both
peer-to-peer (p2p) and client/server topologies.
* Set initial Spring Data GemFire version to 1.7.2.RELEASE, which depends
on Pivotal GemFire 8.1.0.
* Add documentation, Javadoc and samples along with additional Integration
Tests.

Fixes gh-148

samples/httpsession-gemfire-p2p-xml/src/main/webapp/WEB-INF/spring/session.xml

@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<beans xmlns="http://www.springframework.org/schema/beans"
+	   xmlns:context="http://www.springframework.org/schema/context"
+	   xmlns:gfe="http://www.springframework.org/schema/gemfire"
+	   xmlns:util="http://www.springframework.org/schema/util"
+	   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	   xsi:schemaLocation="
+	   	http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
+		http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
+        http://www.springframework.org/schema/gemfire http://www.springframework.org/schema/gemfire/spring-gemfire.xsd
+		http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd
+">
+
+	<!-- tag::beans[] -->
+	<!--1-->
+	<context:annotation-config/>
+	<context:property-placeholder/>
+
+	<bean class="org.springframework.session.data.gemfire.config.annotation.web.http.GemFireHttpSessionConfiguration"/>
+
+	<!--2-->
+	<util:properties id="gemfireProperties">
+		<prop key="name">GemFireP2PHttpSessionXmlSample</prop>
+		<prop key="mcast-port">0</prop>
+		<prop key="log-level">${sample.httpsession.gemfire.log-level:warning}</prop>
+		<prop key="jmx-manager">true</prop>
+		<prop key="jmx-manager-start">true</prop>
+	</util:properties>
+
+	<!--3-->
+	<gfe:cache properties-ref="gemfireProperties" use-bean-factory-locator="false"/>
+	<!-- end::beans[] -->
+
+</beans>