update api for getting USD balance

[dreacot]

The bittrex version 1 API has been deprecated https://bittrex.com/api/v1.1/public/getticker?market=USDT-DCR, and fetching USD balance no longer works.

We should resolve to the v3 API https://api.bittrex.com/v3/markets/DCR-USDT/ticker

[dreacot]

Add binance and kucoin while at it.

[crux25]

@dreacot What do thing about including this functionality in: planetdecred/dcrlibwallet#255

[dreacot]

Sounds good, let's add it

[crux25]

Okay great.