Improvements

Hi!
Stumbled across your grunt plugin - exactly, what i was planning to do for a project. I discovered a bug when building for windows (the nw.exe was removed from the download directory, causing a repeated download of the windows binaries), fixed it and added some changes. 
Amongst them, the build process uses the version and name from package.json, which is trivial for my current project. One can also opt in the timestamp based build folders or use the version from the package.
Feel free to review the changes.

I also have some future ideas to add, e.g. patching the plist file for the mac app to show a correct about window and version number in the finder. The app.icns file could also be copied appropriatly.

What do you think?

Best! jens;