Generate environment variables from links.

Fixes #91.
Yeah, I think the variable names may be wrong:

`man docker-run`
```
   Linking Containers
       The link feature allows multiple containers to communicate with each other.   For  example,  a
       container whose Dockerfile has exposed port 80 can be run and named as follows:

              # docker run --name=link-test -d -i -t fedora/httpd

       A  second  container,  in  this  case called linker, can communicate with the httpd container,
       named link-test, by running with the --link=:

              # docker run -t -i --link=link-test:lt --name=linker fedora /bin/bash

       Now the container linker is linked to container link-test with the alias lt.  Running the  env
       command in the linker container shows environment variables with the LT (alias) context (LT_)

              # env
              HOSTNAME=668231cb0978
              TERM=xterm
              LT_PORT_80_TCP=tcp://172.17.0.3:80
              LT_PORT_80_TCP_PORT=80
              LT_PORT_80_TCP_PROTO=tcp
              LT_PORT=tcp://172.17.0.3:80
              PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
              PWD=/
              LT_NAME=/linker/lt
              SHLVL=1
              HOME=/
              LT_PORT_80_TCP_ADDR=172.17.0.3
              _=/usr/bin/env

       When  linking  two  containers  Docker will use the exposed ports of the container to create a
       secure tunnel for the parent to access.

```