Support id_token validation and discovery flow.

I added the following features.

1. Support state parameter.

2. IDToken Validation
  - http://openid.net/specs/openid-connect-core-1_0.html#IDTokenValidation

3. Discovery
  - http://openid.net/specs/openid-connect-discovery-1_0.html

So the plan is to get a couple other features in and then merge this one. At that point I think this actually deserves a minor version bump rather than a revision bump. Mind changing the version to `0.2.0`? I'll change it myself if need be. Not a huge deal by any means. Will accept this either way.