Now supporting arbitrary payload classes

[BernhardBln]

• The default payload class is now a clojure map
• You can pass any other type as well, but in that case, you will have to
  provide your own (de)serializer and the payload-class when invoking
  service-discovery.
• Clojure maps are converted to Java Hashmaps to get nicer JSON output
• This breaks compatibility with the previous version where String was
  accepted as payload

[BernhardBln]

PS: The only draw-back here is that in the serialised hash map, there is an entry "@Class":"java.util.HashMap", which might break java frameworks that expect certain fields there.

Maybe that could be switched off by using a newer version of the fasterxml object mapper and configuring it ourselves (rather than using the JsonInstanceSerializer from Curator), but I'd rather do that in a separate step :)

[BernhardBln]

I actually dropped the JsonInstanceSerializer in the latest commit, and replaced it by a self-written one using clojure.java.data and clojure.data.json, as the @class attribute caused major problems for us.