-   Make sure that dependency uses a Field.pm method to test for not_blank
    so that can be defined per field.
-  Checkboxes don't make sense being required.
-   Add warnings in addition to errors.  The example would be validating
    HTML where Tidy reports an error or warning that you wish to ignore.
    This would be an extra checkbox next to the warnings.
-   Currently fields call SUPER to validate first, but that sometimes
    generates messages that don't make sense if the error message
    describes the correct format.  Consider PosInteger calling Integer.
-   Docs on all fields.