changed constructor visibility from private to protected in order to be able to derive from Application

closes #88

There is currently a problem if the Admin Client has version 1.2.0 and the Admin Server has version 1.2.1 due to changes to the model. The server is able to handle the old format but the client is not able to handle the new Format. To prevent this problem in future the response is not deserialized. Only the needed inforamtion (the id) is exctracted.

increment version

closes #86

Logfile endpoint available when not using spring.boot.admin.url

Added inner-class for admin client registration configuration which is
conditional on spring.boot.admin.url. Logfile endpoint is now available
when just using Spring Cloud Discovery.

add intellij files to ignore

Add mail notification and event journal

Update README.md

Fix typo

The mail notification is enabled if a mailSender is present specified via spring.mail.* properties.
On default the mail is sent to root@localhost and the chages from UNKNOWN to UP are ignored.

Added a simple event journal, jorunaling all ClientApplicationEvents using a simple, non-persistent store. So all information is lost after restart. Added corresponding view to UI.

The health-check is made at a fixed interval (default: 10s) for all expired status-information (default-lifetime: 30s).
The application health-endpoint must either respond with a JSON-Body having a status-field or with an empty body and all 2xx-http-responses are interpreted as UP and every other status as DOWN.

Fix typo.

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.

closes #74

closes #69

This allows to (re-)set properties via POST-requests to the environment endpoint.

fixes #71