Add HUB_HOST environment variable for Docker networking clarity

[SpencerMalone]

Helps resolve #133 by increasing clarity on how to use the new (Docker v1.7) networking feature.

[ghost]

I would hold off on this. Can we gather some more feedback per #133 discussion please? It may be that in light of new docker network we should expose env variable that were previously hidden by the --link and yes, a bit ugly. We can go with HUB_HOST and HUB_HOST_PORT instead of those cryptic ones auto assigned by --link.

[ghost]

I do see a need to add to README the bits about configuring the grid on multi host machines. Basically Case 1 is covered, now Case 2 and Case 3 scenario can be either added to README or we can start adding pages in wiki section.

[vivanov1410]

ping

[diemol]

I'll close this one since most of its content is covered by #640. I am not sure why this one didn't came through, but anyway thanks to @SpencerMalone and @RubyTester for providing most of the content used in #640.