Generate environment variables from links.

Fixes #91.
It only sets it for the "default" port for a given link. Given that we don't actually allow linking to a container (but just simulate it by linking to an exposed port), it isn't clear which link should be the one that gets that variable.

Docker just picks the lowest exposed port from the host. It would be possible to do that, just based on the alias. But that isn't a straightforward change, and can be implemented later.