Adds Clan Related Handlers and Functions

What this does:
- Adds OnClanAdd() function to UserHandler.cs, SimpleUserHandler.cs, and
AdminUserHandler.cs.
- Adds InviteUserToClan() function to Bot.cs.
- Adds DeclineClanInvite() function to Bot.cs.
- Adds AcceptClanInvite() function to Bot.cs.
- Adds ClanMessages.cs that contains all SteamMessages need for new
actions.
- Filters out Clan Invites from Friend Invites.
- Adds a Clan list that contains all Clans the Bot is in.
- Fixes some spelling errors.
- Updates outdated EEconTradeResponse message types.
Renamed "Clan" to "Group," as per @geel9's request on the original pull-request
Moved group-related messages to their own folder (similar to the TF2GC folder)
Whoops!  Missed a few functions (renaming clan --> group)
Merge pull request #530 from BlueRaja/SteamGroupFunctions

Add functionality for Steam Groups