Test for Issue #472 - IMap containsKey() Does Not Reset Idle Timeout

Add a couple of unit tests to new class IMapTest to verify that the idle timeout is reset when calling both get() and containsKey().
Fix for Issue #472 - IMap containsKey() Does Not Reset Idle Timeout

ConcurrentMapManager modified to update the record's last accessed time when containsKey() is called.
Implemented the InitialMembershipListener
MembershipListenerTest: made sure that the HazelcastInstances are destroyed
Fixes #487.
Merge pull request #480 from hazelcast/stuff

Implemented the InitialMembershipListener
Changed version to 2.6
Updated release notes.
Merge pull request #475 from TheDastardlyRaith/472-imap-containskey-idle-timeout

Fix for Issue #472 - IMap containsKey() Does Not Reset Idle Timeout
Fixes #488.
Updated release notes.
NPE fix.