erltrek_shell messages for dock/undock (#19)
erltrek_ship: add functions for dock/undock (#19)

* Add handle_command({dock}, State)
* Add handle_command({undock}, State)
* Add try_docking(State)
* Adjust indentation of handle_command({phaser,...}, ...)
erltrek_terminal: remove dock/undock event messages
erltrek_shell: fix typo of {dock, docking_complete} (#19)