Kindergarten garden

These are two implementations of the kindergarten-garden exercise.
The first one is somewhat simpler and "normal", the second one is a bit "hackier" and follows the ruby version closer.
The first one makes the plants for each student available via the `plants` method.
The second one creates an attribute for each student who has plants on the window sill.

While being a bit more complex, the second version also is more interesting to implement IMO…

What do you think?

(I can of course squash the commits if necessary.)

Links:
[Exercise description](https://github.com/exercism/x-common/blob/master/kindergarten-garden.md)
[Ruby implementation](https://github.com/exercism/xruby/tree/master/kindergarten-garden)