Use rate.Limiter for throttling

[Rukenshia]

from my manual testing, this will no longer cause high CPU usage (as
reported in #72).

I wasn't sure how to properly test the throttling, but looking at the documentation of the rate package this should do.

I used this for testing:

package main

import (
	"log"
	"time"

	"github.com/Rukenshia/trello"
)

func main() {
	for {
		log.Printf("Creating 100 new clients")
		for i := 0; i < 100; i++ {
			client := trello.NewClient("foo", "bar")

			log.Printf("client: %v", client)
		}
		time.Sleep(5 * time.Second)
	}
}

before this change, CPU usage went up with every 5 second iteration. After the change, the CPU usage is consistently at 0%.

[adlio]

Thank you for the contribution Rukenshia!