JSON::Schema::Loader
Wedge JSON::Schema::Loader into place in Validator

Conflicts:
	lib/json-schema/validator.rb
Document JSON::Schema::Loader in the README

Prolly got some textile syntax a bit wrong, and the link to the yard
docs won't work prior to this being merged.
Revert some unnecessary changes

I'll submit a separate PR to add pry (and pry-byebug) as dependencies in
the Gemfile.

Reverted some changes I made to attributes/ref.rb that would conflict
with #186.
Simplify reset of schema_loader in tests
Rename Loader -> Reader
Merge pull request #175 from pd/schema-loader

JSON::Schema::Reader