Add ability to configure adapter

This changeset generalizes the usefulness of this adapter. Instead of
hard-coding the bootstrapping namespace, it can be configured in the
karma config as follows:

```js
module.exports = function(config) {
  config.set({

    ...

    frameworks: ["cljsTest"],

    client: {
      cljsTest: {
        main: "my-tests.namespace",
        entry: "run-all-the-karma-tests"  // optional: defaults to `run`.
      }
    },

   ...
  })
};
```

In the example given above, the adapter expects the find the following:

```clj
(ns my-tests.namespace
  (:require [clojure.string :as string]
            [cemerick.cljs.test :as test]))

(defn get-total-test-count []
  (reduce + (map count (vals @test/registered-tests))))

(defn run-karma-test! [test-name test]
  (let [log-output (atom '())
        start (.getTime (js/Date.))]
    (test/set-print-fn! (fn [x] (swap! log-output conj x)))
    (let
        ;; [{:result result :time time} (run-test-with-time test)]
        [result (test/test-function test)]
      (.result js/__karma__
               (clj->js
                { "id" ""
                  "description" (-> test-name name)
                  "suite" [(-> test-name namespace str)]
                  "success" (and (zero? (:error result))
                                 (zero? (:fail result)))
                  "skipped" nil
                  "time" (- (.getTime (js/Date.)) start)
                  "log" [(string/join "\n" (reverse @log-output))]})))))

(defn ^:export run-all-the-karma-tests []
  (do (.info js/__karma__ (clj->js {:total (get-total-test-count)}))
      (doseq [[ns ns-tests] @test/registered-tests ]
        (doseq [[test-name test] ns-tests]
          (run-karma-test! test-name test)))
      (.complete js/__karma__ (clj->js {}))))

```

TODO: Add docs.