tests: Fix Nicira MPLS pop action test
core: Add hack to initialize core for tests

Some tests were failing because they expected core to be initialized,
which, as of 333fe0f6b, has to be done explicitly.

This isn't a good fix, but works for the moment.
addresses: Add IPAddr6 class for IPv6 addresses

This is part of experimental IPv6 support and partially addresses
issue #35.
nx: Use IPAddr6

This augments the IPv6 support in the nicira module to use the new IPv6
address class.

This is part of experimental IPv6 support and partially addresses
issue #35.
packet: Add IPv6 support

This includes some ICMPv6, neighbor discovery, AAAA DNS records, etc.

This is part of experimental IPv6 support and partially addresses
issue #35.
packet: Add some mDNS stuff
addresses: Add 'raw' properties

These should replace to toRaw() methods which can now be considered as
deprecated.