Support "claims" OpenID Connect auth request parameter #20

OpenID Connect Core 1.0, Section 5.5, claims parameter:

OPTIONAL. This parameter is used to request that specific Claims be returned. The value is a JSON object listing the requested Claims.

An example Claims request is as follows:

{
  "userinfo":
  {
      "given_name": {"essential": true},
      "nickname": null,
      "email": {"essential": true},
      "email_verified": {"essential": true},
      "picture": null,
      "http://example.info/claims/groups": null
  },
  "id_token":
  {
      "auth_time": {"essential": true},
      "acr": {"values": ["urn:mace:incommon:iap:silver"] }
  }
}