issue-71

added jmx module, making use of statistics from Terracotta S3 repo
added StatisticsMBeanRegister  (user facing)
added stats observers for put/get/removal/eviction
missing stats gathering

If this exception ever gets thrown it's due to a bug in this code.  We could maybe init the cause of the assertion error to be the exception though, that way it's more easily diagnosable if it gets thrown.