Added law for Monoid.isId.
Precised law structure for semigroup/monoid/group action.
Added a law for groups with a left and right action.
Merge pull request #325 from denisrosset/topic/group-laws

Topic/group laws
Merge commit '5572c1d910b8b7a5b75d49aa6b5bdcdaa39ca219' into topic/order-laws