Support WebSocket URL

[caicaishmily]

Can we support the WebSocket URL to receive the event messages pushed by contracts?

[alisinabh]

Ethers relies on Ethereumex by default as the underlying RPC Client implementation and it does not yet support WebSocket transport. The issue for that is tracked here if you want more info: mana-ethereum/ethereumex#91

[alisinabh]

This has now changed. Ethers needs to add functions related to websockets