add pure ruby version of litecoin scrypt.c

  https://github.com/litecoin-project/litecoin/blob/df06b04cf7096bce9f46fbfd81b6d79c5723be42/src/scrypt.c
make litecoin scrypt a bit more memory friendly
cleanup scrypt a bit more
Merge branch 'master' of https://github.com/lian/bitcoin-ruby