curried Either and added Left and Right

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

The implementation you are suggesting (`Left` or `Right`, inheriting from `Either`) is a different approach from the current Ramda. Imo currying is not applicable in that case, because a curried `Either` is a `Left` and will not be able to become a `Right` (or you must swap the values).

I suppose a `Left.prototype.of` returns a `Left` and a `Right.prototype.of`and `Either.prototype.of` a `Right`.

`Right.prototype.map` should decide to return either `Left` of `Right` based on the return value of `f`.

Do we want to proceed that way? I like the conceptual elegance of have either `Left` or `Right`, but the elegance and simplicity of the current implementation are appealing too.