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- Updates outdated EEconTradeResponse message types.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 did the clan messages this way, as down the road I could see problems when SteamKit2 updates, and erases Clan messages.