Let's write a feature: Traversable for cats library

cats (https://github.com/funcool/cats) is a library with category theory and algrebraic abstractions for Clojure. It implements abstractions like Semigroups, Monoids, Functors, Applicatives and Monads for Clojure and ClojureScript built-in types and provides useful types like Maybe or Either.

In this meetup we'll implement the Traversable (https://github.com/funcool/cats/issues/50) abstraction together in what's known as Mob Programming (http://haikuco.de/blog/2015/on-mob-programming/). At the end of the meetup we'll have learned about sharing code between Clojure and ClojureScript with reader conditionals, extension of built-in types, testing, Clojure idioms and some useful functional patterns.