Json migrator

In order to be able to migrate the json configurations of dashboards,
this change adds a way to easily get access to the fields. The
implementaion makes it easy to extend the provided helpers in the future
to change other fields or tables as well.

Example usage:

    Migrator.run Migrator::Widget do |widget|
      widget['foo'] = widget['bar']['baz']
    end

@juliusv @stuartnelson3 Have a look. Implementation might be a bit too fancy. I had too much fun in the subway. The actual migration interface is pretty clean at least ;)

Solves #112.