New rules disallowGuardClause and validateGuardClause.

Guard clause is a very useful trick to make our code more elegant. So I thought jscs may need some rules about guard clause. I had finished disallowGuardClause and validateGuardClause. But requireGuardClause is still in progress, it is a little difficult to detect all situations which can be refactored into guard clause. 
please revert this and the changes made to require-curly-brace.