added Windows support for reset_shots_folder method

On Windows, you can't call FileUtils.mkdir method right after FileUtils.rm_rf because of NTFS "pending deletion" state (cf. http://stackoverflow.com/questions/8487290/is-ruby-rm-rf-asynchronous#answer-8490780 & http://support.microsoft.com/?id=159199).
As a workaround, we just have to deleted the directory content instead of the directory itself.