Always return the PrepararedStatement object that is cached internally

JAVA-263 #fixes
Merge branch '1.0' into 2.0

Conflicts:
	driver-core/src/main/java/com/datastax/driver/core/Cluster.java
	driver-core/src/main/java/com/datastax/driver/core/Session.java
	driver-core/src/main/java/com/datastax/driver/core/SessionManager.java