Add framework support for customizable serialization.

The new Serialization framework supports PDX Serialization as a possible configuration option for SSDG in addition to the already existing use of GemFire/Geode's DataSerialization framework.

Resolves Issue #2.

samples/boot/gemfire/src/main/java/sample/server/NativeGemFireServer.java

@@ -157,7 +157,7 @@ public final class NativeGemFireServer implements Runnable {
 		regionFactory.setEntryIdleTimeout(newExpirationAttributes(1800, ExpirationAction.INVALIDATE));
 
 		Region region = regionFactory.create(
-			GemFireHttpSessionConfiguration.DEFAULT_SPRING_SESSION_GEMFIRE_REGION_NAME);
+			GemFireHttpSessionConfiguration.DEFAULT_SESSION_REGION_NAME);
 
 		return gemfireCache;
 	}