Fix read only fields lookup of endpoint field

I have an `endpoint` field and it breaks when I try to make it readonly because model base get the `self.endpoint` instead of `instance.endpoint`.
Fix a NameError to sqlalchemy.sql.expression.literal_column

This file use the `literal_column` expression, but import the `literal`.
Merge pull request #82 from iurisilvio/patch-2

Fix read only fields lookup of endpoint field
Merge pull request #83 from iurisilvio/patch-3

Fix a NameError to sqlalchemy.sql.expression.literal_column
Remove readonly fields from fields tuple before create model_form

It solves the problem with sqla, I don't know if the other backends have the same issue.

If you pass readonly fields in `only` parameter, WTF try to update these fields and breaks because they are not defined.
Merge pull request #84 from iurisilvio/patch-4

Remove readonly fields from fields tuple before create model_form
Merge remote-tracking branch 'upstream/master'