Update currency list in BalanceResponse

[RoanBrand]

IMO opinion all amounts and prices must be kept as strings. I have run into many problems with using float64 or even big.Float when working with money. This is the same reason the API uses strings.

I also added currencies that were in the API docs response example. I realize there is even more currencies on the exchange now that they haven't added to the docs. This needs to be changed to a dynamic response map[string]string or []string{symbol, balance} so that all currency changes will just work, and not to mention easier for users to handle.

I need this, so will be changing along as I need on my forked version. I can make additional future PRs if you want.

[beldur]

Thanks for your input Roan! I get your point and think the idea is not far fetched. Would it be possible for you to write some down some example's where this is problematic? If we do this breaking change, we should document the reasoning behind it here in this thread.