Implement get_buy_orders and get_sell_orders methods

[peerchemist]

Both can just be a front for existing get_orders method.

[peerchemist]

Bittrex allows querrying of only sells or only buys:

type | required | buy, sell or both to identify the type of orderbook to return.

https://bittrex.com/api/v1.1/public/getorderbook?market=BTC-LTC&type=both