Adding update task for upgrading grunt and npm tasks.

During the lifetime of a project, there are likely going to be updates to our grunt toolchain.

This adds a `yo red-django:update` command which will update npm dependencies, the main `Gruntfile.js`, and all the grunt configurations in the `/grunt` directory.

This will read the existing project's `package.json` and add or update module versions to the latest versions specified in [generator-red-django/app/templates/_package.json](https://github.com/ff0000/generator-red-django/blob/master/app/templates/_package.json).

Because of the way yeoman handles conflicts when generating files, you will be able to see the changes file by file and approve or reject them one at a time.

If you have made changes to the grunt configurations for your project, you will need to resolve those conflicts yourself.