Merge pull request #870 from supar/add-column-default-attribute

Add column default attribute
Revert "Add column default attribute"
Merge pull request #881 from astaxie/revert-870-add-column-default-attribute

Revert "Add column default attribute"
Skip add DEFAULT if the field is in relations (rel or reverse)
Add column DEFAULT attribute. Do not add if field is key or in
relations.