Merge attributes by concatenating :class and overriding :id

Hi,
    This pull request deals with conflicts in class and id attributes. Class attributes are concatenated and ID attributes are replaced:

    [:div.foo {:class "bar"} "baz"] =>  <div class="foo bar">baz</div>
    [:div#foo {:id "bar"} "baz"]    =>  <div id="bar">baz</div>

(This supersedes [pull request 57](https://github.com/weavejester/hiccup/pull/57) which I've now closed)