Add GyazMail.app v1.5.16

Thank you for the new cask.

The homepage URL is redirecting to `http://gyazsquare.com/gyazmail/`, is there any particular reason why you left out the `/gyazmail/`?

```
$ curl -I http://gyazsquare.com/
HTTP/1.1 302 Found
Date: Sun, 07 Dec 2014 18:13:04 GMT
Server: Apache/1.3.42 (Unix) PHP/5.2.6 mod_ssl/2.8.31 OpenSSL/0.9.8za
Location: http://gyazsquare.com/gyazmail/
Content-Type: text/html; charset=iso-8859-1
```

The `homepage` stanza should be pointing to application homepage, not developer's homepage, if possible.
https://github.com/caskroom/homebrew-cask/blob/master/doc/CASK_LANGUAGE_REFERENCE.md#required-stanzas