Smarter pkg

We are presently using `grunt.file.readJSON` to [read in](https://github.com/linemanjs/lineman/blob/da7b69982e44220bdd107798d86abc78749dc125/config/application.coffee#L7) package.json. Since `file.readJSON` isn't package.json-aware, the `pkg` object is a direct representation of the package.json file. I propose we use npm's [`read-package-json` module](npm/read-package-json) to read the file. This way, the `pkg` object will be normalized according to how npm sees the package. This will allow npm defaults to shine through, and various short-form properties (like `author`/`contributors`) to be parsed and expanded.