lib: add get_marketdata()

get_marketdata() can get at most 50 traded data of all coins

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>
market_watcher: to monitor the market

market_watcher will do the pseudo trade of the market data,
so it can tell you which coin is worth to do auto trading

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>