use worker threads to make ajax requests, introduce cljs futures, and eliminate jquery dependency

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castra uses jquery for its (a) ajax fn and (b) promise. this dependency can be removed by (1) using the native XMLHttpRequest (2) making requests from a worker thread wrapped in a clojurescript future abstraction.

(defn xhr [{:keys [url credentials headers body]}]
  (doto (js/XMLHttpRequest.)
        (.open "POST" url false)
        (set-req-headers headers)
        (aset "withCredentials" credentials)
        (.send body)))

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ref http://www.html5rocks.com/en/tutorials/workers/basics/#toc-inlineworkers