Added property allowChallenges to User

Added property allowChallenges to User to block / allow challenges. Property is set true or false with chat commands.
Added blockchallenges and allowchallenges

Added commands /blockchallenges or /bc and /allowchallenges or /ac to let the user choose whether to accept challenges from other users or not.
Merge pull request #137 from Joimer/patch-1

Added /blockchallenges and /allowchallenges so users can block challenges