Introduced ApplicationContext/Boot

Servers are started with dedicated *Application
classes that create an application context and
start their respective servers via Boot.

https://github.com/markfisher/xdzk/issues/13
Updates from review, fixed tests

Changes that came from the PR review:
https://github.com/markfisher/xdzk/pull/17

Modified the AdminContainerObserverTest to work with the new
application context (as opposed to a static singleton).