[WIP] JSON Deserializer

Targets #151

This is highly WIP. Please review the interface definitions before I
start implementing this stuff!
Yes. From the example on the yajl homepage, it seems that you _have to_ pass each option on its own and say "I want to set this" with passing the `1` in the arguments. Completely stupid, if you ask me, but the example does it this way (also because in the example, you can set the options via cmdline).

Things can be optimized later, first this stuff should work.