curried Either and added Left and Right

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

@donnut Sorry, that was entirely an aside triggered by @buzzdecafe's "i implemented the types a while ago because i wanted to learn them."  It wasn't any attempt to answer the question.  I do tend to ramble.

I don't have a strong opinion about this.  I see arguments on both sides.  It's a lot easier to maintain ones we roll ourselves and it's easier to make sure they fit with our philosophy.  But if there are tricky things to work out and it's difficult to get them right or get them performant, there's a great deal to be said for taking advantage of someone else's expertise.  So? I'm wishy-washy, so what?

One of the most important points, though, is that Ramda should work well with any [FantasyLand][fl] implementation.  Once that's true, users should be able to work with whatever one we choose to roll in to our larger builds -- our own or a third party's -- or whatever other one they prefer.

  [fl]: https://github.com/fantasyland/fantasy-land