curried Either and added Left and Right

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

> This doesn't solve the issue you raised with a f failing or returning null.

I know, which is why i suggested punting to the user: "`f` must be a function that returns a value that can be wrapped in a `Right`". 

I'm not convinced this is correct. But it compares well with other impls. 

* https://github.com/fantasyland/fantasy-eithers/blob/master/src/either.js
* https://github.com/folktale/data.either/blob/master/lib/either.js