Incorporate latest changes from WebDAV-Push

[ArnyminerZ]

Added:

• PushPublicKey: An abstract class that provides the logic for all public key properties:
  • ClientPublicKey
  • ServerPublicKey
• AuthSecret (for PushPublicKey)
• WebPushSubscription: now also has clientPublicKey and authSecret
• PushResource (for WebPushSubscription, replaces the String property)
• PushTransport an interface of Property that tags all the push transports (right now only WebPush)
• WebPush: comes from the simpler property of web-push, but now has an optional ServerPublicKey

Modified:

• Changed namespace to https://bitfire.at/webdav-push
• PushMessage: Now has propStat instead of topic
• PushTransports: transports is now a set of PushTransport

Renamed:

• PushSubscribe -> PushRegister

[sunkup]

How about unit tests for the properties like PushPublicKey which have some more methods. Not sure if everything is covered in WebPushTest, so having seperate test classes per property might make sense, no ?

Or does it not make sense to add exhaustive tests yet, since it's still all experimental?

Maybe also a question for @rfc2822

Looks okay otherwise, though I am unsure whether everything is working correctly without test coverage which could be inspected.