Fix for #62, #63, #64: automatically detect schema URL

This patches add automatic schema URL detection to `bin/validate-json`.

Schema is detected from:
- "$schema" property in the root object
- "profile" property in the Content-Type header (once #61 has been implemented, see http://json-schema.org/latest/json-schema-core.html#anchor34)
- "describedBy" link header (once #61 has been implemented, see http://json-schema.org/latest/json-schema-core.html#anchor35)

This pull requests builds on:
- https://github.com/justinrainbow/json-schema/pull/70
- https://github.com/justinrainbow/json-schema/pull/71
- https://github.com/justinrainbow/json-schema/pull/72
- https://github.com/justinrainbow/json-schema/pull/73
- https://github.com/justinrainbow/json-schema/pull/74