trades: implement trades requesting

[odeke-em]

Fixes #3

• Implement a LastTrades method that retrieves
  trades since a certain ID as provided by okcoin's
  REST API at https://www.okcoin.com/rest_api.html#spapi

Exhibit:

package main

import (
  "fmt"
  "log"

  "github.com/orijtech/okcoin/v1"
)

func main() {
  client, err := okcoin.NewDefaultClient()
  if err != nil {
    log.Fatal(err)
  }

  tres, err := client.LastTrades(&okcoin.LastTradesRequest{
    Symbol:      okcoin.BTCUSD,
    LastTradeID: 354425297,
  })
  if err != nil {
    log.Fatal(err)
  }

  fmt.Printf("Last Trades for Symbol: %s\n\n", tres.Symbol)
  for i, trade := range tres.Trades {
    fmt.Printf("#%d ID: %d\n", i, trade.ID)
    fmt.Printf("\tAmount: %.3f\n", trade.Amount)
    fmt.Printf("\tPrice: %.3f\n", trade.Price)
    fmt.Printf("\tType: %s\n\n", trade.Type)
  }
}

which gives

Last Trades for Symbol: btc_usd

*0 ID: 354425531
  Amount: 0.061
  Price: 4615.820
  Type: buy

since I just used that request trade for requesting
for a previous test set.