added getGroups action, getUsers fix

[sercinci]

The add-on on the community has been marked as "Unmaintained".
https://xenforo.com/community/resources/xenapi-xenforo-php-rest-api.902/
Tried to contact @Contex by email, after no reply I've decided to implement by myself the getGroups action.

Request:
api.php?action=getGroups

Examples:
api.php?action=getGroups

Response:
[{ "user_group_id": 1, "title": "Unregistered / Unconfirmed", "display_style_priority": 0, "username_css": "", "user_title": "Guest", "banner_css_class": "", "banner_text": "" }, { "user_group_id": 2, "title": "Registered", "display_style_priority": 0, "username_css": "", "user_title": "", "banner_css_class": "", "banner_text": "" }, { "user_group_id": 3, "title": "Administrative", "display_style_priority": 1000, "username_css": "", "user_title": "Administrator", "banner_css_class": "", "banner_text": "" }, { "user_group_id": 4, "title": "Moderating", "display_style_priority": 900, "username_css": "", "user_title": "Moderator", "banner_css_class": "userBanner bannerHidden", "banner_text": "" }]

[Contex]

I sadly do not have time to work on the addon as much as I would like to, it has almost become a job.
I have replied to numerous emails regarding XenAPI the last couple of weeks, I apologize if I missed yours. XenAPI is more of a hobby project of mine, which should ideally be rewritten one day.

I will take a look at your PR some day if I get the time for it.