(HI-14) Implement structure traversal lookup

Previously hiera would lookup a key as the name of a first level key
in the backends. Now, if you're interested in a value contained inside
a hash (of any depth) you can simply pass all the keys concatenated
with dots. For example:

    $ hiera user
    {"name"=>"hiera", "home"=>"/home/hiera"}

    $ hiera user.name
    hiera