Add SSTableSplitter tool to split sstables offline

patch by slebresne; reviewed by krummas for CASSANDRA-4766
Merge branch 'cassandra-1.2' into cassandra-2.0.0

Conflicts:
	src/java/org/apache/cassandra/db/compaction/CompactionTask.java
Merge branch 'cassandra-2.0.0' into cassandra-2.0