Add grunt tasks to ease packaging

This commit introduces
- a grunt multi-task (`atbuild`) that allows to simply re-package aria templates source code. It is also used internally for the production build of the framework.
- a grunt multi-task (`easypackage`) that allows to create packages for an aria templates-based application. It simplifies the configuration that is generally needed in order to achieve this goal by exposing a simple API to the developer.

These tasks are now available in any project including the framework. The necessary files are published to npm along with the framework and there is no need to explicitly add atpackager or noder-js as dependencies in order to execute them.
The reason why `outputDirectory` doesn't use the `getPath` method in the second object is because that setting becomes the default output directory for a build of the framework. If someone installs ariatemplates in their project and makes a custom build without specifying an output directory, I want the output of the build to end up in the root `build\target` folder of their project