JSON parser

Here's a first attempt at a JSON parser. Notes:

* It probably needs documentation.
* This quickly proved to be a bit fiddly when I first tried to use [https://github.com/joshbuddy/jsonpath](jsonpath). I abandoned that strategy, but jsonpath might still be useful when we get to needing to map JSON data in the DSL.
* I'm not sure if the behavior in `Krikri::JsonParser#get_child_nodes` is exactly desirable - it was an attempt to "sanely" handle the fact that JSON natively has an array datatype, unlike XML. 
* I rearranged the parser specs; there maybe a more idiomatic way to do this in rspec land, so feedback there is welcome too.