IE-100: Kafka:o-c-a: topic and message-key attrs

JIRA: https://jira.spring.io/browse/INTEXT-100

Add `topic(-expression)` and `message-key(-expression)` attributes to avoid upstream configuration
to specify them in the `MessageHeaders`

Polishing for XSD

Conflicts:
	spring-integration-kafka/src/main/java/org/springframework/integration/kafka/support/KafkaProducerContext.java
	spring-integration-kafka/src/main/java/org/springframework/integration/kafka/support/ProducerConfiguration.java
	spring-integration-kafka/src/main/resources/org/springframework/integration/config/xml/spring-integration-kafka-1.0.xsd
	spring-integration-kafka/src/test/java/org/springframework/integration/kafka/config/xml/KafkaOutboundAdapterParserTests.java

INTEXT-100: Add `Assert.notNull(this.evaluationContext);`

INTEXT-100: Fix parser potential NPE

INTEXT-100: Add `KafkaHeaders`

Merge branch 'INTEXT-100-1' of ..\spring-integration-extensions into INTEXT-100
A null topic is allowed - see `KafkaProducerContext L 52`.