Create a publisher compliance test.

[julien51]

This test should accept any url and perform the following test:

• It should include a self link header
• It should include a hub link header
• The designated hub should allow for subscriptions
• The designated hub should allow for unsubscriptions

There are more tests but I'm not sure how we can handle them:

• The hub should ping the subscribe when the resource updates
• The hub should check with the publisher if the subscriber can subscribe

[pfefferle]

that would be very helpful!