factor out loading cluster health

new cluster service is responsible for loading stuff like this.
display meaningful message in case the health status could not be loaded
remove superfluous logger
replace Node.Factory with NodeService

Models shouldn't load themselves if possible
remove never used controller
isProcessing moved to Node from JVM stats

changed loading of buffer info to Node class
changed loading of server jvm stats to ClusterService
resume/pause moved to Node class
Merge git://github.com/Graylog2/graylog2-web-interface into develop