This will help to get rid of repetitive class casts, and make
PacketCollector api more inline with itself (since some methods
are already generic return methods).
Similar flaw was also fixed in 2c7f1efe80.
Also ensure that OfflineMessagesManager doesn't leak collector by using
a try/finally block.
Fixes SMACK-592
instead of using the old baseName=smack appendix=project.name approach,
we are now going convention over configuration and renaming the
subprojects directories to the proper name.
Having a prefix is actually very helpful, because the resulting
libraries will be named like the subproject. And a core-4.0.0-rc1.jar is
not as explicit about what it actually *is* as a
smack-core-4.0.0-rc1.jar.
SMACK-265
