Open issues for major release

[tvdijen]

• [x] Upstream blocking issues in xml-security
• [x] AttributeValue must be able to deal with XML-structures as a value (i.e. Dutch national ID requires an EncryptedID there, but also EPTiD) (#340)
• [x] BaseID currently cannot be encrypted because it doesn't implement the encryption interfaces
• [x] ~~Method getEncryptionBackend for any encryptable element is currently implemented to return null. Needs further implementation,~~ or are we OK with the default?
• [x] SignedElementTestTrait needs to move to xml-security
• [x] Many setter-methods are still 'public' and should be migrated to 'protected'
• [x] We have to manually set $this->dataType = C::XMLENC_ELEMENT in the constructor of an encryptable element. It would be much cleaner if we could set a constant instead of 'abusing' the constructor for this. Will probably require changes to xml-security EncryptableElementTrait
• [x] We need a solution for the HTTPRequest binding.. It's currently not possible to migrate it to the new signing interface
• [ ] We're not passing the container's blacklisted algorithms anywhere
• [ ] grep -i TODO and fix those
• [ ] Drop the Certificate/ directory and use the similar classes from xml-security
• [ ] Rewrite the files in the Configuration/ directory. Create Configuration-classes for IdentityProvider and ServiceProvider with public getters/setters that can be used by implementations using this library.
• [x] The ecp:Request element has not yet been implemented (https://github.com/simplesamlphp/saml2/pull/341)
• [x] The ecp:RelayState element has not yet been implemented (https://github.com/simplesamlphp/saml2/pull/341)
• [x] The ecp:SubjectConfirmation element has not yet been implemented (https://github.com/simplesamlphp/saml2/pull/341)