cqlsh: add support for multiline comments

patch by Aleksey Yeschenko; reviewed by Brandon Williams for
CASSANDRA-5798
Merge branch 'cassandra-1.2' into cassandra-2.0.0

Conflicts:
	CHANGES.txt
	bin/cqlsh
Merge branch 'cassandra-2.0.0' into cassandra-2.0
Merge branch 'cassandra-2.0' into trunk