fixes #230

This commit contains multiple changes:
 - Start the registration task after the ApplicationReady event. This makes
   AdminClientProperties.isReady() unnecessary.
 - The RegistarApplicationListener has two new public methods to start/stop the
   periodic registration task.
 - Add spring.boot.admin.auto-registration property (default: true). When this
   property is set the registration task is automatically scheduled after the
   ApplicationReadyEvent.

closes #151

In case prefer-ip is true and no server.address is set use the IP address from
InetAddress.getLocalHost(); like it is done in Eureka.

There is a problem with the relaxed binding for system environment variables in
spring boot and ignoreUnknownFields for ConfigurationProperties. So we need to
remove it for now.

fixes #135

Take the servlet-path into account when computing the management-/healht-url.
This is not done for the service-url, since the main use-case for setting the
servlet-path is to get the dispatcher-servlet out of the way for other servlets.

fixes #133

By using local.server.port, local.managment.port and ApplicationReadyEvent the port detection and ready state detection can be simplyfied in boot 1.3

 * Use @EventListener instead of ApplicationListener-interface
 * Don't inherit ClientApplicationEvent from ApplicationEvent - so JournaledEvent isn't needed anymore
 * added eclipse formatter rules
 * reformatted all files

In conjuntion with server.address / management.address this plays more nicely with interfaces with multiple ipAdrresses. Also remove spring.boot.admin.client.useAddressOf.

fixes #112

If `spring.boot.admin.client.useIpAdressOf=<network-interface>` is set properly the ip-address of the specified network-interface is used in the inferenced url to register with.
In case the address cannot be determined an exception is thrown and the client won't register.
This is for example useful in Docker environments making the inferenced urls work.

Checks for the presence of Ssl in ServerProperties and that it is
enabled, uses https in that case for client URL. Added test case for
option.

fixup! Add optional automatic deregistration on shutdown - turned off by default; only periodically try to register when the context is active

This change not only allows you to use `context-path` and `contextPath` but also respects automatic port assignment when using `server.port=0` or `managment.port=0`.
Also corrects the value if server.port != mangement.port and server.context-path and management.server-path is set.
fixes #57

server.context-path not used when application registers itself #32
Typo in About #31

This allows you to protect your spring-boot-admin rest api.
Supporting basic auth on management-endpoints needs more work.

* 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.