Accept field instances as field type declartion

[turicas]

If we accept field instances we can support easily customize fields' behavior without needing to subclass, like:

table = rows.import_from_csv('filename.csv',
        force_types={'data': rows.fields.DateField(input_format='%d/%m/%Y')})

We may force all received items (in force_types and fields) to be Field instances (not classes).