Merge pull request #45 from tacone/master

Add support for HasOne relations
deleting associed files and cleanup related without constraint
better explanation for back()
better explanation for back()
fix,  the field name should be always $this->name and not $this->db_name
centralize attributes()  to widget and "merge" instead override
demo fo hasOne  in dataedit demo
shortcut attr()
do not insert duplicate HasOne, fixes #46