$ref support

[delvedor]

As titled.
This implementation supports both internal and external definitions.

Caveats:
If the user is using an external json file for his definitions it must use __dirname to get the correct path.
See the following example (the same of the test).

const schema = {
  title: 'object with $ref',
  type: 'object',
  properties: {
    obj: {
      $ref: __dirname + '/ref.json#/definitions/def'
    }
  }
}

If everything is ok to you I can update the docs.

[mcollina]

Good work, one minor issue on the use of require.

[mcollina]

Never do this, it's sync and done in a place where it might cause to bugs. We should not load files into Node. However, we should use an approach similar to https://www.npmjs.com/package/is-my-json-valid#external-schemas.

[mcollina]

Nice!!

[mcollina]

Great work!!!!

[mcollina]

LGTM