Add build tasks for Windows fat binary gems for use with the RubyInstaller.

atomic is the only dependent gem for rails-4.0, that requires the RubyInstaller DevKit in order to install the gem. It would be nice to allow an installation without DevKit by providing fat binary gems for x64-mingw32 and x86-mingw32 targets.

If you don't want to suffer with windows build stuff, I could also do the build and provide the resulting gem after each release. I do this for several other projects already.