Create :jnt reader conditional support

[arrdem]

This changeset has one primary change: supporting the :jnt conditional clause in addition to :clj.

#?(:cljs "nope" :jnt "w00t" :clj "nope")
;; => "w00t"

#?(:cljs "nope" :clj "still works")
;; => "still works"

Incidental changes to this end include:

• Upgrading to Java 1.8
• Providing an interface default implementation of clojure.lang.Seqable/iterator (Java 1.8 feature)
• Adding clojure.lang.RT/union [set, coll] -> set (used in the reader changes)

Fixes #68

[arrdem]

Needs reader tests...

[arrdem]

:shipit: