GCM mode: Optimize GCM speed with pre-computed tables.

Tables take 64KByte per each key.

Encryption performance is more than doubled
(29 MBps vs 8MBps for AES128).

As a drawback, key setup is much slower (1300 key/s
on the same machine).

[dlitz@dlitz.net: Replaced MacMismatchError with ValueError]
[dlitz@dlitz.net: Replaced ApiUsageError with TypeError]
[dlitz@dlitz.net: Included changes from the following commits from the author's pull request:]
- [9c13f9c] Rename 'IV' parameter to 'nonce' for AEAD modes.
- [ca460a7] Made blockalgo.py more PEP-8 compliant; The second parameter
            of the _GHASH constructor is now the length of the block
            (block_size) and not the full module.
[dlitz@dlitz.net: Whitespace fixed with "git rebase --whitespace=fix"]