convert ruby struct to clojure map

This adds a Clojurize protocol implementation for org.jruby.RubyStruct. I would have used Struct#marshal_dump instead of iterating over the entries, but it seems that JRuby doesn't implement that method.

I needed this for a library I'm writing. I don't know if this patch is desired but I thought I should share it since converting structs to maps seems like a fairly common requirement.