Ipv6 support

Adding ipv6 capability in the interest of test support for https://github.com/datastax/python-driver/pull/144.
This should be useful for other dev stacks as well.

Notes:
- This should not change ipv4 behavior at all
- To run with ipv6 requires C* with https://github.com/apache/cassandra/commit/4a9634a31ea7bbdf3b6c0173a94e6ab3911bc989 (merged to 1.2, 2.0, 2.1 at time of writing; no tag yet)
- To run with ipv6 requires python-drvier with (a superset of) the changes posted in the pull request listed above. (one way to test ahead of release is to export CQLSH_NO_BUNDLED=1 to break out of the C* distribution, and point to a local driver version using PYTHONPATH)