Add '$schema' property into root object.

[IvanGoncharov]

According to JSON Schema spec:

When extending JSON Schema with custom keywords, schema authors SHOULD define a custom URI for $schema.

$schema value could be http://swagger.io/v2/schema.json#

It simple feature which enhance JSON Schema compatibility.
For e.g. it will allow automatic validation of Swagger file by JSON Schema validator.
Also it will motivate creation of schemes for Swagger extension.

On the negative side, it partly duplicate info from swagger field.
Which isn't a big problem, because it occur once per Swagger file.

[IvanGoncharov]

Duplicate of #496