Remove scalaz dependency.

Hey folks,

Talked to @brapse yesterday. We don't really use any scalaz magic in this project. `\/` (read: "disjunction") is essentially right-biased `Either`. There are some subtleties with using `Either` in a `for`-comprehension ([link](https://issues.scala-lang.org/browse/SI-5793) for the interested). Anyway, this PR removes scalaz from project dependencies and rewrites all the disjunctions with `Either`.

/cc @typingduck @johnglover @seeemilyplay