Fix incrementNonce() to work when multiple bytes need to increment.

[causalnet]

This fixes #9 and has the incrementNonce() code that more closely matches KeepassXC's, properly handling multiple byte increments.

[causalnet]

This matches what libsodium's utils.c does more accurately.

The original C code:

for (; i < nlen; i++) {
        c += (uint_fast16_t) n[i];
        n[i] = (unsigned char) c;
        c >>= 8;
    }

goes through each byte, incrementing if there is overflow in the previous byte. The Java code in this PR should do the same thing.