Headers gain 3 bytes of reserved zeros

[Jason A. Donenfeld]

I did it. White paper updated. Protocol webpage updated.

Benefits:
- Natural 4 byte alignment of ciphertext, which confers significant
speedups on many platforms
- Natural 4 byte alignment of resultant plaintext IP packet; similar speedups.
- Natural integer alignment of index and nonce; similar speedups for
packet lookups.
- Easier to implement in hardware.
- Protocol is more extendable.

Detriments:
- 3 bytes less in the MTU.

This will be part of the next snapshot.