Add test services.

Add implementaion for some DAO methods and test them.
Remove _class field to be saved in the database, because this is not good for application design.
Change saveDocument implementation.

Mongo is smart enough to insert new object when save(..) is invoked and objectId is null.
So save can be used both for insertion/creation.
Merge branch 'master' of https://github.com/polinankoleva/docmag