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

settings.gradle

@@ -8,6 +8,10 @@ include 'samples:findbyusername'
 include 'samples:hazelcast'
 include 'samples:hazelcast-spring'
 include 'samples:httpsession'
+include 'samples:httpsession-gemfire-clientserver'
+include 'samples:httpsession-gemfire-clientserver-xml'
+include 'samples:httpsession-gemfire-p2p'
+include 'samples:httpsession-gemfire-p2p-xml'
 include 'samples:httpsession-xml'
 include 'samples:rest'
 include 'samples:security'
@@ -15,4 +19,5 @@ include 'samples:users'
 include 'samples:websocket'
 
 include 'spring-session'
+include 'spring-session-data-gemfire'
 include 'spring-session-data-redis'