Support linking containers via containers yaml config

Please consider this PR.

It is an attempt to solve my separated web and data (postgresql) containers [problem](https://meta.discourse.org/t/bootstrap-web-only-container-gets-pg-error-for-separated-data-container/20632/) that I have asked in meta discourse.org.

i.e. connect to pg from the application: `docker run --name app --link pg:data -d application-that-uses-postgres` Or, connect to it via psql: `docker run -it --link pg:data --rm postgres sh -c 'exec psql -h "$POSTGRES_PORT_5432_TCP_ADDR" -p "$POSTGRES_PORT_5432_TCP_PORT" -U postgres'`
etc.

It works on my setup:
  - Linux 3.13.0-32-generic # 57 -Ubuntu SMP Tue Jul 15 03:51:08 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
  - Docker version 1.2.0, build fa7b24f

Hope this is useful.

Thanks.