Override method to get wallet balance

[mpetrunic]

By default getBalance method will query node every time it's called which generates too many api calls and it's prone to failure.
I would propose that wallet subscribes to balance updates and keeps cached balance. This would allow us to have balance healthcheck without generating api calls.

I think listener is something like:

provider.on(wallet.address, function(newBalance) {
    console.log(newBalance);
});

We should verify that it uses websocket and not polling with infura