Advanced Search - Postal code range searching is missing

When Criteria.php was refactored, the postal_code_low and postal_code_high fields got dropped unintentionally. They should be re-added to the form and tested to make sure they work. (Looks like the search logic was retained in Contact/BAO/Query.php so this may be just a matter of adding the fields back into the the form and testing).

Also need to fix range query so that it differentiates numeric range searches (e.g. US Postal Code ranges) from alphanumeric searches. This will insure that searches on "numeric" postal codes with leading zero's work properly.