Raise source compatibility to 1.7 (Java7)

Now that Java 8 is released, it's time to raise Smack's source
compatibility. Let's also use some of the "new" Java 7 features like
"catch multiple".
Flush dnsjava cache if no results are returned

This should prevent some cases in mobile scenarios, where an DNS
lookup on a unavailable data connection results in an empty result
which is cached by dnsjava.