Fix bad ref test for proxy support

If you run behind a proxy, you won't get a socket error, you'll get an
OpenURI::HTTPError instead (because the proxy will return a 503).  That's
perfectly acceptable, too.
Another allOf ref test, and rename class
Pulled nested classes out of json-schema/validator.rb

Error classes, schema:validator and attribute
Removed duplicate code for accessing validators, adding a new
Got rid of version_string_for and metaschema_for, in favour of having
validators nominate their own names and metaschemas
Merge pull request #130 from iainbeeston/refactor_slightly

Refactor classes and validator accessor methods
Merge pull request #124 from mpalmer/fix-bad-ref-test

Fix bad ref test for proxy support
Merge pull request #126 from mpalmer/all-of-ref-test

Another allOf ref test, and rename class
Adding Matt Palmer to contributors