Return more specific types (e.g. Collection → List)

be generic as possible in what you accept, but more specific in what you
return.

Also tweak MultiuserChatManager methods a bit.
Improve javadoc of PingManager.setPingInterval()
Don't use dynamic dependency for dnsjava
Smack 4.1.0-alpha6
Smack 4.1.0-alpha7-SNAPSHOT
Merge remote-tracking branch 'upstream/master'