Add a first pass at generating a better directory structure.

I had a go at implementing directory structure generation, as per [issue 39](https://github.com/thoughtbot/liftoff/issues/39), submitted here for feedback.

There are a couple issues at the moment:

* Not all files are automatically moved from Supporting Files into Support; `InfoPlist.strings` isn't returned by [`PBXGroup`'s `files` method](http://rubydoc.info/gems/xcodeproj/Xcodeproj/Project/Object/PBXGroup#files-instance_method) so it doesn't get transferred.
* The files created by Xcode's templates, like the `xcassets` file, are just left where they are.

I think it may be worthwhile to [implement a `liftoffrc` file](https://github.com/thoughtbot/liftoff/issues/47) before I'd consider this feature to be complete — it would be useful to be able to override Liftoff's directory structure inside there.