Fix RefResolver and make it compatible with draft-04

[jojo1981]

The current implementation of the RefResolver can only handle JSON schema definitions which result in a tree and not a graph and only to a certain depth.

The RefResolver needs to be replaced or fixed so it can handle JSON schema definitions which contains cycles.

I'm still working on a solution for the RefResolver and I'm almost done. When I've added some regression tests I will submit my code and create a pull request.

[mfn]

This would be much appreciated. We tried to add swagger json schema validation do our pipeline in PHP using this library but ultimately failed because of the draft-04 requirement.

[jojo1981]

OK, then I will try to speed things up.

[jojo1981]

Trying to get it also working with older PHP versions (I'm using PHP 5.6.14) and make sure the travis build: https://travis-ci.org/jojo1981/json-schema will be OK before creating a PR. So for now it's still work in progress.

[jojo1981]

@mfn please join the review.

[bighappyface]

@jojo1981 please go ahead and open a PR for your work. I basically agree that the public can decide.

[bighappyface]

Derp... nvm: #245