Implement default identity provider using postgres

[aeneasr]

This will be part of #62

I want hydra to be able to maintain a list of identity providers which can be added and removed using the CLI:

hydra idp add https://identities.foobar.com/
hydra idp remove https://identities.foobar.com/

Hydra should ship a default/exemplary identity provider using postgres located here. Identity providers must implement an HTTP REST interface with (at least) the following capabilities:

Mandatory (used by Hydra):

• GET /identities: Should return all known identities. Should support pagination.
• GET /identities/<id>: Should return an identities information
• POST /authenticate: Should return an identitiy if valid credentials are given. Question: What to support id/password, username/password, email/password, ... ?

Recommended: Not used by Hydra but useful in a real world scenario:

• POST /identities: Should create a new user
• DELETE /identities/<id>: Should delete a user
• PUT /identities/<id>/password: Should update the identities password

Optional features:

• Password reset
• ... ?

Identities should be sent using JSON:

{
    "id": "12341234" // mandatory
    // ...
}

Errors should be sent using the HTTP status header and a JSON message:

{
    "error": "Not found",
    "description": "some reason"
}

For inspiration check out the current account module

[aeneasr]

superseded by #62