Add functionality for Steam Groups

I noticed that [this commit](https://github.com/Jessecar96/SteamBot/pull/271) was never accepted because there were merge conflicts, and @geel9 wanted "clan" renamed to "group."  I fixed the conflicts and renamed the functions.

Note that with this pull-request, existing `UserHandler`s will need to handle the `OnGroupAdd()` callback in order to compile.