Added grunt shell and task to run nw from node_modules

Since `nodewebkit` is now a dev dependency, I figured we would be better off by creating a grunt task for developers to run from the install in the node_modules directory.

This will save the hassle of explaining now to install node webkit, and it will ensure the intended version of nodewebkit is being used to run the source.

I'm not sure the executable paths for windows or linux yet, but we'll need to add those. I put minor platform logic at the top of the Gruntfile. At first I was thinking to extract it to another node module, but that's pretty much what `os` is.