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>
Rework beatrix and server tests to use base class GuicyKillbillTestSuiteWithEmbeddedDB
Merge branch 'master' of github.com:killbill/killbill