Issue 74: ensure Java 6 compatibility

Vaadin 7 requires officially only Java 6, so it'd be beneficial if this
great plugin also had that minimum requirement.

Replacing the try-with-resource in ResourceBundleMessageProvider I
noticed that the stream would not be closed if the encoding was unknown,
so I fixed that, too.

Didn't touch the samples.
Merge pull request #87 from tomaswolf/java6_compatibility

Issue 74: ensure Java 6 compatibility