GORA-389: Document or List mapping change cause NPE in clearDirty()

This close PR #15

Squashed commit of the following:

commit f07ac202b1890afd27d5e071dff5eafdc34bc044
Author: Damien Raude-Morvan <drazzib@drazzib.com>
Date:   Sat Nov 1 12:16:38 2014 +0100

    GORA-389 Add MongoStore specific tests for Map/List handling

commit 75805e2eaf3c1c69573dc1b2b2df12a25aec8d25
Author: Damien Raude-Morvan <damien@dictanova.com>
Date:   Wed Oct 8 20:03:36 2014 +0200

    Don't perform clearDirty() multiple times.

commit d7a6a4794f2fef492f1e116e1108b59d8e9c81bb
Author: Damien Raude-Morvan <damien@dictanova.com>
Date:   Wed Oct 8 20:04:06 2014 +0200

    Return empty DirtyListWrapper or DirtyMapWrapper instead of null.