Add teleport platforms

* Teleport platforms are an alternative to traditional boarding.
* Activate teleport platforms to transfer player to adjacent spaceship.
* Fix "look" command behaviour.
* Accessors: tile/item/mob <-> interior <-> spaceship <-> exterior, this
has to be organized and enforced.

Commit references #3.
Teleport from one platform to another

* Enforce and organize relationships between classes: exterior <->
spaceship <-> interior <-> tiles/items.
* Add mock objects to tests/ directory.

Commit closes #3.
Merge branch 'teleport' into develop