message: fix and extend CBitcoinExtKeyBase template
  fix Decode call (req. only one param)
  add constructor for base58c->CExtKey
  coming from btc@7cb1f9f7eb8162a792b4b87bba99fa21c682582e
diff: @@ -167,7 +167,7 @@ public:
  K GetKey()
  {
  K ret;
  - ret.Decode(&vchData[0], &vchData[Size]);
  + ret.Decode(&vchData[0]);
  return ret;
  }
  @@ -176,6 +176,10 @@ public:
  SetKey(key);
  }
  + CBitcoinExtKeyBase(const std::string& strBase58c) {
  + SetString(strBase58c.c_str(), Params().Base58Prefix(Type).size());
  + }
  +
  CBitcoinExtKeyBase() {}
  };
  