Add Created, Started, and Finished time fields to Torrent

[justinrush]

This PR adds the Created, Started, and Finished fields to the Torrent struct, closing #16 .

Example usage:

s := rtorrent.New("https://rutorrent.myhost.com/RPC2", false)
torrents, err := s.GetTorrents(rtorrent.ViewSeeding)
if err != nil {
	panic(err)
}

for _, torrent := range torrents {
	fmt.Printf("Name: %s; Created: %s; Finished: %s; Started: %s\n", torrent.Name, torrent.Created.Format(time.RFC3339), torrent.Finished.Format(time.RFC3339), torrent.Started.Format(time.RFC3339))
}

Example output:

Name: Some.Torrent.With.All.The.Dates; Created: 2021-03-16T16:35:32-05:00; Finished: 2021-04-07T13:12:53-05:00; Started: 2021-04-07T13:10:00-05:00
Name: Some.Torrent.With.No.Create.Date; Created: 1969-12-31T18:00:00-06:00; Finished: 2021-04-10T09:07:04-05:00; Started: 2021-04-10T09:01:43-05:00

Note that the tests in general fail with and without these changes, however, the test that actually gets the torrent(s) works. I am not sure if the torrent used in the test even exists anymore.

[coveralls]

Pull Request Test Coverage Report for Build 230

• 17 of 23 (73.91%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 49.654%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rtorrent/rtorrent.go	17	23	73.91%

Totals
Change from base Build 204:	0.6%
Covered Lines:	430
Relevant Lines:	866

---

💛 - Coveralls

[justinrush]

added a newer torrent and now tests pass

[mrobinsn]

Thank you @justinrush for the PR, released in https://github.com/mrobinsn/go-rtorrent/releases/tag/v1.6.0