Add ability to set/get logging levels to nodetool
patch by Jackson Chung reviewed by Vijay for CASSANDRA-6751
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/service/StorageService.java
	src/java/org/apache/cassandra/tools/NodeCmd.java
	src/resources/org/apache/cassandra/tools/NodeToolHelp.yaml