Features/jsonpayload

I've added a subset of JSONPath support to heka to close #309.  I only support traversing down children (.) and the subscript operator ([]).  

We *can* add full JSONPath support, but we'll need to do something quite a bit fancier than what I do now.  We'd also need to resolve what to do with JSONPath results where the return value is a blob of JSON.  I'm not entirely sure that makes sense in the context of heka, but I could be convinced otherwise.