depth method
Order book depth from /v1/depth
https://github.com/binance-exchange/binance-official-api-docs/blob/master/rest-api.md#order-book
Implementation
Future<BookDepth> depth(String symbol, [int limit = 100]) =>
_public('/v1/depth', {'symbol': '$symbol', 'limit': '$limit'})
.then((r) => BookDepth.fromMap(r));