Initial generator

This one is ready to go. Let me know if everything looks good and I should jump on a next one.

This uses a newly published module called [grunt-prompts](https://npmjs.org/package/grunt-prompts). It is essentially just the built-in prompts of `grunt-init` with a few helpers. At the moment it is very `grunt-init` specific but could become more generalized to better fit in as a yeoman module instead. Happy to do that at any time.

Another note, I previously had the dependency versions automatically being determined using [latest](https://npmjs.org/package/latest). I removed it and went to hard coded versions instead. It is fast if the module is already in the npm cache... but on the first time it has to fetch them with `npm view` thus makes it a bit slow. Here is the [removed code](https://github.com/shama/generator-node/blob/eba07959430de06141e1744e242a6d25f0ed54e4/app/index.js#L110). Not sure on your guys' thoughts about automatically determining the latest versions of dependencies.

Thanks!