Refactor classes and validator accessor methods

[iainbeeston]

There are a few things that have been bugging me in the json-schema codebase, that I thought I could clean up. Here's a summary:

Before	After
json-schema/validator.rb defined several classes: errors classes, JSON::Schema::Validator as well as JSON::Validator	I've pulled these out to their own files
The code for getting a validator (from JSON::Validator.validators[]) was duplicated in a number of places, including the internal url format used by JSON::Validator to store schemas	I've made an accessor method that takes a uri string instead
JSON::Validator included the methods for getting the version string and metaschema for a validator, making it harder to add new validators	I've made each validator now know about it's own version strings and metaschema instead

Hopefully this will make the code easier to understand, and make future changes easier.

[hoxworth]

Awesome! I'll take a look over this later today along with all the other issues that have accumulated, but at a cursory glance this looks like a lot of stuff I've been wanting to do for a while...

[hoxworth]

👍 Looks great, thanks!