Add Dogecoin support

Please note this depends on https://github.com/lian/bitcoin-ruby/pull/133

With apologies for the false start earlier, this has now been run through a complete sync twice (and is up at http://toshi.jrn.me.uk:5000/ ). There's some tweaks to the transaction cache loading code which improve performance on blocks with extreme numbers of transactions, which also fixes the problem of the Dogecoin testnet being infeasible to sync.