Convert message brokers to use new sensor feeds

- For ActiveMQ, Qpid and RabbitMQ
- Converts ActiveMQ from groovy to java
- Converts from groovy to java: JMSBroker, Queue, MessageBroker
Sensor feeds: just warn if try repeated feed.stop()
Sensor feed JsonFunctions: avoid NPE

- In cast(Class), return null if input is null
Convert JmxNotificationFilters from groovy to java
Merge pull request #446 from aledsage/refactor/groovy-to-java-20130104

Refactor message brokers: groovy to java, and new sensor feeds