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.

It works on mine 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.