Corrected logic in blank/nil validations.

Corrected the logic in the allowed-to-be-blank functions to properly allow nil values when allow-nil is true, but allow-blank is false. Previously, both allow-blank and allow-nil had to be set to true to allow nil values due to clojure's blank? function returning true for nil.
Merge pull request #15 from WesleyJohnson/master

Corrected logic in blank/nil validations.