add checkpoints for mainnet blocks 300000 and 305000 and testnet block 250000
eliminating redundant attribute definition
refactoring storage to remove Sequel related code from StoreBase
p2sh support, multikey sign
cleanup p2sh support in Builder, proper tests
optimize Bitcoin::Validation::Block.spent_outs_txins to avoid calling get_tx on TxIns
clean up Bitcoin::Validation::Block.spent_outs_txins optimizations