Adds Clan Related Functions Fixes #178

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- Updates outdated EEconTradeResponse message types.

I added Clan messages to SteamBot due to me seeing some problems down the road when SteamKit2 updates and erases Clan messages.