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.
Two remarks in code below:
- outputDirectory uses getPath in first subobject, and plain target in the second subobject, we should keep consistency
- should sourceDirectories be a string?