AssociatePublicIpAddress support for goamz/packer

Provides required underlying AssociatePublicIpAddress support for packer, goamz PR required as a dependency - https://github.com/mitchellh/goamz/pull/22

Ignore my testing/revert testing/debugging verbosity commits, files changed shows the relevant stuff at least.

Includes respective website updates for documentation, unit tests may still need updating though.

Relates to mitchellh/packer#578