Add IWithMeta protocol to RAtom

[frerom]

Fixes #257

[danielcompton]

This would return a new instance of the Ratom when calling with-meta. This seems idiomatic amongst other core Clojure code, but I'm not sure what its effects on Reagent would be, both because Reagent makes heavy use of metadata, and because object identity is quite important in Reagent, and could lead to confusing scenarios for rerunning dependent changes. It might be fine, but this should probably be checked out further.

[Deraen]

I think this will be safe. Reagent doesn't change RAtom meta ever, so only case where new RAtom is created using with-meta should be when user does so. And I think it won't be easy to do that accidentally, unless one has two nested RAtoms or such.

(deftest ratom-with-meta
  (let [value {:val 1}
        meta-value {:meta-val 1}
        state (r/atom (r/atom value))]
    (is (= (meta @state) nil))
    (swap! state #(with-meta % meta-value))
    (is (= (meta @state) nil))   
    ))