Added Callbacks and new Validations

[adelowo]

I have added two new callbacks beforeValidation and afterValidation : The latter would only run if all validations was sucessful

    $fileUploader->addCallback('beforeValidation', function (FileUpload\File $file
    ) {
        //about to validate the upload;
    });

    $fileUploader->addCallback('afterValidation', function (FileUpload\File $file
    ) {
        //Yay, we got only valid uploads
    });

I also splitted the Simple Validator into two (MimeType and SizeValidator) since sometimes the mimetype might only be what should be validated. And the SizeValidator also accepts a min value, all this were not in the simple validator ;

I also edited the Validator interface a bit since the validate method had a param $tmp_name that wasn't used (was redundant).

I also edited the setMessages method by allowing the default message to be over-riden/changed by the user, array_merge was used initially but it always didn't reflect the new error messages set by the user since it incremented the error array count. That said, a constant is used to define the error message in the validate method meaning the new error messages would never be shown.

I also wrote Unit Tests for the new validators