Some work towards a (very slow) GCM implementation

Does not currently work due to some error in ghash, likely a
misunderstanding of the field representation.
Working GCM. Add support for AEAD modes in self tests
propagate from branch 'net.randombit.botan.aead-modes' (head 9f75a9bcacc77447b2916104f0ccbabd2c93b516)
            to branch 'net.randombit.botan' (head 9e94d89357c08ffc5f7cb5cc3e86c755da96ca8e)
Relnote GCM support
Add get_aead in lookup.h
Support for non-CBC modes in ciphersuite list gen