replace render-html with protocol

Hi James,

during load-testing our hiccup based page we noticed a whole lot of lock contention for the multimethod lock of `render-html`. As the lock contention is only fixed in Clojure 1.5 (see here: http://dev.clojure.org/jira/browse/CLJ-988), I changed the multimethod to a protocol-based `render-html` that works just the same without the locking.

Gerrit