Add Snapshot Create API

[wedneyyuri]

This PR adds support for Elasticsearch's snapshot API.

Example program:

package main

import (
	"context"
	"time"

	elastic "gopkg.in/olivere/elastic.v5"
)

func main() {
	client, err := elastic.NewClient()
	if err != nil {
		panic(err)
	}

	response, err := elastic.
		NewSnapshotCreateService(client).
		IncludeGlobalState(true).
		Indices("index1").
		Name(time.Now().Format("2006-01-02-15-04-05")).
		Repository("repo1").
		WaitForCompletion(true).
		Do(context.TODO())
	if err != nil {
		panic("error during elasticsearch snapshot create")
	}

	if response.Snapshot.State != "SUCCESS" {
		panic("elasticsearch snapshot state != SUCCESS: " + response.Snapshot.State)
	}
}

Based on #508

[wedneyyuri]

I didn't notice the generate-api branch, I will make some changes and reopen when done.

[wedneyyuri]

Reopened by #533 (another branch)