curried Either and added Left and Right

properties are prepended with _ to allow functions left() and right()
isRight() added to pass strict mode

Don't get me wrong, but isn't it a better idea to fork the monads from one of the other repos, like you listed? If we can not 'add any value' with a better implementation/functionality, forking would save work. (To refute myself, the others don't have a curried Either ;)