Add support for DELETE ... IF EXISTS to CQL3

Patch by Tyler Hobbs, reviewed by Sylvain Lebresne for CASSANDRA-5708
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/statements/ModificationStatement.java
Merge branch 'cassandra-2.1' into trunk