fix topology.metrics.consumer.register validation: Map, not String
The topology.metrics.consumer.register is a list of hashes, but its schema was StringsValidator. Added a 'MapsValidator' (list of Map) and fixed the schema.
LoggingMetricsConsumer dumps metrics to log file
* Added a LoggingMetricsConsumer example of a MetricsConsumer. Use it by adding 'conf.registerMetricsConsumer(LoggingMetricsConsumer.class);' when you construct your topology.
* Added a dedicated appender to the example logback/cluster.xml called METRICS.
  Its log format omits the class and priority since with the dedicated appender those are redu
Error if opaque value's txId is ahead of batchTxId
Merge pull request #636 from infochimps-labs/opqval_error_checking

Throw Exception if OpaqueValue's txId is ahead of batchTxId
update changelog
Merge pull request #635 from infochimps-labs/metrics_logging_consumer

LoggingMetricsConsumer, to dump metrics to log file
Merge pull request #634 from infochimps-labs/metrics_validation_fix

fix topology.metrics.consumer.register validation: Map, not String