specify balances in state in big number string so it can handle larger amount

[swswsw]

balances in state is currently a js number. it has limitation on how large the number can be.
we cannot store js BigNumber in state, doing so cause various problems.
so we should store the number as string in the state.
everytime we use it, we should remember to convert it to bignumber.
evertime we store balance in state, we should remember to convert it to string representation.

allow user to use string to specify amount in input so big number can be used.

eg.

curl -X POST http://localhost:3000/txs -d '{"type": "bet", "marketId": "market2", "outcome": 1, "amount": 10, "user": "5wvwWgKP3Qfw1akQoXWg4NtKmzx5v4dTj"}'

change the amount to string.

curl -X POST http://localhost:3000/txs -d '{"type": "bet", "marketId": "market2", "outcome": 1, "amount": "2000000000000", "user": "5wvwWgKP3Qfw1akQoXWg4NtKmzx5v4dTj"}'

everytime we read the amount, we should use
let amount = new BigNumber(inputAmount);
to make sure we convert it to bignumber.