Detect schema automatically

[cweiske]

validate-json should try to detect the schema automatically:

• "$schema" property in the root object
• "profile" property in the Content-Type header (once Validate URLs #61 has been implemented, see http://json-schema.org/latest/json-schema-core.html#anchor34)
• "describedBy" link header (once Validate URLs #61 has been implemented, see http://json-schema.org/latest/json-schema-core.html#anchor35)