add a kafka-extension sample

extensions-samples/kafka/jersey-pubsub/README.md

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+This sample assumes Apache Kafka 
+For more information on Kafka, refer to http://kafka.apache.org/documentation.html
+If Kafka is running at its default host and port localhost:9092, you will need to adjust
+parameter bootstrap.servers in a configuration file specified by the init paramter
+org.atmosphere.kafka.propertiesFile.
+
+Run this sample twice with this command lines :
+- 1st instance : mvn jetty:run-war -Djetty.port=8080
+- 2nd instance : mvn jetty:run-war -Djetty.port=9090
+
+You can use your browser to connect to either http://localhost:8080 or http://localhost:9090 and
+observe the messages posted to the same topic are broadcasted to its subscribers.

extensions-samples/kafka/jersey-pubsub/pom.xml

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+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <parent>
+        <groupId>org.atmosphere</groupId>
+        <artifactId>atmosphere-samples-project</artifactId>
+        <version>2.5.0-SNAPSHOT</version>
+        <relativePath>../../../</relativePath>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>atmosphere-kafka-demo</artifactId>
+    <packaging>war</packaging>
+    <name>atmosphere-kafka-demo</name>
+    <properties>
+        <extensions-version>2.5.0-SNAPSHOT</extensions-version>
+    </properties>
+    <build>
+        <defaultGoal>install</defaultGoal>
+        <plugins>
+            <plugin>
+                <artifactId>maven-war-plugin</artifactId>
+                <version>2.4</version>
+                <configuration>
+                    <overlays>
+                        <overlay>
+                            <groupId>org.atmosphere.samples</groupId>
+                            <artifactId>atmosphere-jersey-pubsub</artifactId>
+                        </overlay>
+                    </overlays>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.atmosphere</groupId>
+            <artifactId>atmosphere-kafka</artifactId>
+            <version>${extensions-version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.atmosphere.samples</groupId>
+            <artifactId>atmosphere-jersey-pubsub</artifactId>
+            <version>${project.version}</version>
+            <type>war</type>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.geronimo.specs</groupId>
+            <artifactId>geronimo-servlet_3.0_spec</artifactId>
+            <version>1.0</version>
+            <scope>provided</scope>
+        </dependency>
+    </dependencies>
+
+</project>