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.
workaround fix for tests on Windows

without this fix, I get the following error when running "rspec"
  1) Wraith When comparing images
     Failure/Error: Given { WraithManager.reset_shots_folder }
     Errno::EACCES:
       Permission denied - ./shots/test/test1.png
     # ./lib/wraith_manager.rb:26:in `reset_shots_folder'
     # ./spec/wraith_spec.rb:10:in `block (2 levels) in <top (required)>'

sounds like the "shots" folder don't get reseted before the "compare" test
added Windows requirements instructions to README file