Merge pull request #13 from allenxwang/master

Register the rule as a monitor object to expose the available servers count metric for AvailabilityFilteringRule.
Gradle Release Plugin - pre tag commit:  'ribbon-0.1.7'.
Gradle Release Plugin - new version commit:  'ribbon-0.1.8-SNAPSHOT'.
Merge commit '49c14abb1b22733aeb6ab19d322d5852770ec9f7'