Adding IPv6 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 found this confusing. I think we're missing some context. If I understand correctly, the `--fixed-cidr-v6` parameter tells the Docker daemon to use IPv6 addresses in the specified range. The next parameter, `--global-ipv6`, tells a container to use an IPv6 address from the previously specified range. Can you please clarify the relationships between all three of these parameters as you introduce them? Thanks.