Update GridFieldBulkImageUpload_Request.php

Silverstripe 3.1.0 requires statics to be defined as private

http://doc.silverstripe.org/framework/en/3.1/changelogs/3.1.0
Merge pull request #21 from MattyBalaam/patch-1

Update GridFieldBulkImageUpload_Request.php
FIX #22 update static variable to private
update compatibility to SS >=3.1
SS 3.0 compatibility branch notice
Readme update
Merge commit '1c744fc6bb0793f272513a138c5b2d3fdce77e1e'