Add Validation::uploadedFile()

Add a simple wraper function for the various file validation methods.
This method makes it easier to wrap up the various file validators into
one method.

Refs #4456
Make uploadError() and uploadedFile() accept missing files.

This makes validating forms where there is an optional file much easier.
Make missing files pass validation when optional flag is used.
Merge pull request #4524 from cakephp/validation-upload-file

Validation upload file