server: expose injector from KillbillGuiceListener

Use the injector to access instances from server tests, similar to what we do
in other modules.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
server: add missing OSGI and usage modules in tests

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
First pass at OSGI / JAX-RS integration

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>