Generate environment variables from links.

Fixes #91.
Docker sets one other variable named `ALIAS_PORT=protocol://addr:port`. 

We should probably add that for completeness.

```
root@54d5a05b9e5a:/opt/webapp# env | sort
DB1_ENV_PG_VERSION=9.3
DB1_NAME=/web/db1
DB1_PORT=tcp://172.17.0.15:5432
DB1_PORT_5432_TCP=tcp://172.17.0.15:5432
DB1_PORT_5432_TCP_ADDR=172.17.0.15
DB1_PORT_5432_TCP_PORT=5432
DB1_PORT_5432_TCP_PROTO=tcp
HOME=/
HOSTNAME=54d5a05b9e5a
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/opt/webapp
SHLVL=1
TERM=xterm
_=/usr/bin/env
```