Redesigned registration.

* registry-key is derived from URL
* option to specify a management-URL and other various config options see spring-boot-starter-admin-client/README.md
* less chatty registration
* some more test cases
* show application URL in overview and headline
* show application info in overview
* set HTTP-Codes in RegistryController

fixes #13
fixes #16
closes #17
closes #18

I didn't add the key-value-properties to the Application. Since it was somehow redundant to the /info-endpoint.