Added group methods.

Added various group methods. I used SteamBot's implementation as a reference since I couldn't find any other docs for this.

**New methods:**
* `groupInvite(steamIdGroup, steamIdInvited)`
  * invites user to a group you are member of
* `acceptGroup(steamIdGroup)` and `declineGroup(steamIdGroup)`
  * accepts / declines group invite; can be used in conjunction with `group` event

`steamIdGroup` has to be in `steamID64` format [[example](http://steamcommunity.com/groups/tradingcards/memberslistxml/)].