IPv6 support

[jsommers]

Support for IPv6 is minimal through the framework. Some work on packet header classes has been done, but has been tested in only limited ways. Interface class doesn't support ip6 addresses, and doesn't presently support multiple addresses of any kind.

[jsommers]

version 1.0.0 has support for necessary ICMPv6 capabilities for neighbor discovery, etc., and router discovery, etc. test coverage needs improvement, and ip6 router exercises need to be written still.