fix show_if with belongs
fix unique validator with date/time fields
settings is_gae in autocomplete
always appends IS_NOT_IN_DB
Fix default validator when unique=True

Append the IS_NOT_IN_DB validator so it is applied to the final value after any previous validators may have altered the original value.
Merge pull request #467 from ilvalle/fix_show_if

fix show_if with belongs
Merge pull request #469 from abastardi/issue/unique_validator

Fix default validator when unique=True