Added missing methods and fixes

[BirknerAlex]

This merge request contains following changes:

• Added method to list memberships by organisation ids
• Added method to show many tickets
• Fixing search method for tickets.

Working search for tickets example:

search := fmt.Sprintf("brand:%v created>%v status<closed",
	c.Zendesk.BrandId,
	time.Now().Add(-time.Hour*48).Format("2006-01-02"),
)
result, err := zendeskClient.SearchTickets(search, &zendesk.ListOptions{
	Page:    0,
	PerPage: 100,
})

[eggman64]

Thanks so much for your contribution!

Could you add tests for the two new methods. You can refer to the existing tests, or reach out in case you need some help