Handle KeyboardInterrupt properly in cqlsh

Patch by Jordon Pittier, reviewed by brandonwilliams for CASSANDRA-5481
Merge branch 'cassandra-1.2' into cassandra-2.0
Merge branch 'cassandra-2.0' into cassandra-2.1.0
Merge branch 'cassandra-2.1.0' into cassandra-2.1

Conflicts:
	CHANGES.txt