Change ManyToMany between entry & tag

Change ManyToMany between entry & tag

Following https://gist.github.com/Ocramius/3121916

Be sure to remove the related entity when removing an entity.

Let say you have Entry -> EntryTag -> Tag.
If you remove the entry:

 - before that commit, the EntryTag will stay (at least using SQLite).
 - with that commit, the related entity is removed