SP metadata

Implementation of SP Metadata.
- Produces the bare mininum metadata required by the spec.
- Fixes #4.
Discussed with @albinsunnanbo . `Issuer` is not a good name as the Idp is not the issuer in this case. It is the SP that issues the `AuthnRequest`. Changing to `EntityId` would be confusing too as that is a property of any SAML2 participant. When used as a property name on an `Idp` object it makes sense, but not on its own.

We'll change the query string param to `idp` which makes it clear that we are referencing and Idp.

Use idp.