allow to connect Wallet to remote node
Currently we can create Wallet instance only by instruction
node.attach_wallet(). There is some support for connecting to mainnet node api (via
main_net singleton). It might be useful also to connect wallet to mainnet node (or testnet node running on other computer). We only need to somehow specify ws endpoint while creating Wallet object instance (possible also http endpoint in future, there is related enhancement hive#168).