Log in to npm when using the release task

@matt-gadd commented on [Fri Sep 02 2016](https://github.com/dojo/grunt-dojo2/issues/34)

The task currently checks whether the user is logged in, and that the user has permissions to publish the package. It might be worth adding the capability to pass credentials to the task and it logging in automatically (as well as logging out after the release is completed).