Potential fix for Issue 125

I've got a day's knowledge of Git/hub, and I've only been programming in Clojure for a couple of weeks, so I'm probably doing this wrong.  But, maybe this will be somewhat helpful. This includes my fix.  

I've added a new :client-param named :cookie-spec-factory.  If you set this to a value, the add-client-params! function will register the factory and set the policy name to the name of the cookie spec factory class name.  I felt this was a good way to ensure you don't register over another already registered factory. If you don't set the :cookie-spec-factory, it defaults to CookiePolicy/BROWSER_COMPATIBILITY for backwards compatibility.

Here's an example of how I used this new code:

```clojure
(def easy-cookie-spec (proxy [BrowserCompatSpec] []
                        (validate [cookie origin] nil)))

(def cookie-spec-factory (proxy [CookieSpecFactory] []
                           (newInstance [params] easy-cookie-spec)))

(defn login [cookie-store]
  (client/post "http://a.url.com" {:form-params { ;;;;;;whatever form params you're setting
                                                                                 }
                                                                   :cookie-store cookie-store
                                                                   :client-params {:cookie-spec-factory cookie-spec-factory}
                                                                   }))
```

Also, regardless of whether you accept this, I'd love some feedback on what I'm doing wrong here (whether it's with git/hub or with clojure).  Thanks!