feature: implement GET /photos/:id/statistics

[hbagdi]

No description provided.

[ketankr9]

But that is already implemented as link, right?

// Statistics return a stats about a photo with id.
func (ps *PhotosService) Statistics(id string, opt *StatsOpt) (*PhotoStatistics, *Response, error) {
...
	endpoint := fmt.Sprintf("%v/%v/statistics", getEndpoint(photos), id)

And also currently there isn't any api with /photos/:id/stats, maybe deprecated, can you please verify? link

// Stats return a stats about a photo with id.
func (ps *PhotosService) Stats(id string) (*PhotoStats, *Response, error) {
...
	endpoint := fmt.Sprintf("%v/%v/stats", getEndpoint(photos), id)

[hbagdi]

The response on stats and statistics are different.
The endpoint is deprecated and unsupported now and we should remove it from this library as well.