Added support for grunt-node-webkit-builder

I added the config and copied over the modified node-webkit build to build/cache so it won't download it again. Maybe a symlink would have been better but I'm not sure if it will work on windows, os x and linux. To build PopcornTime, simply run `grunt nodewebkit` and it will put the builds into `build/releases`.

The Windows build is still only the (more or less) raw files, not a single packaged .exe. Maybe some installer creation system should be used for it. Because InnoSetup-setups can't be build on linux/mac, maybe NSIS should be used. That would make it easier to use a Build-Server lateron.