With this commits the server no longer depends on the client. The
classes for the clients are moved to de.codecentric.boot.admin.client
package.
Since the perspective on applications is totally different from client
and server the model has been split apart.

What means this for users:
1) The client is no longer a transitive dependency for your admin server
you may need to add it explicitly to your dependencies, respectively
jolokia-core in case you're using discovery and want to use the jmx-bean
interface

2)In case you have cutomised parts of the client you need to reflect the
package change.

fixes #332

Add spring.boot.admin.client.enabled property to explicitly disable the
spring-boot-admin-client, (e.g. for unit tests). Defaults to true. The client is
active when spring.boot.admin.client.enabled is true and spring.boot.admin.url
is set.

fixes #156