Adding IPv6 network support to docker daemon

This PR adds IPv6 support to the docker daemon.
Features:
- adding CLI flags `--ipv6` and `--fixed-cidr-v6`
- Setting the bridge IPv6 to `fe80::1/64`. This IP will be used by the containers as gateway.
- registers the fixed-cidr-v6 in the ipallocator

Missing:
Test and Documentation.
Follow-up PR for enabling the container to use IPv6. (I have the code but still have to clean it up a bit and create a PR).
I wanted to add them but #8896 popped up and I thought it would be a good idea to provide my code so we can compare and pick the best of both.

**Update 2014-11-04**
- Documentation is improving
- Container IPv6 Implementation is included in this PR as a separate commit
I think this whole introductory section can be cut down and shortened. It's not really the job of Docker docs to explain what IPv6 is. I'd prefer to see an introductory sentence or two and then the links to resources for learning more (wikipedia, IETF spec, etc.).