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
Again, I'm not sure we need all the explanatory/introductory material above. This is Docker documentation, not a tutorial in IPv6 conventions.