Forward compatibility with Guava 16.0.1.

With these changes, tests pass with Guava 14.0.1, 15.0, 16.0.1.
This allows applications to version manage Guava to a more recent
version if they need recent Guava features.

With this pull request Ostrich requires at least Guava 14 since
the Stopwatch.elapsed() method does not exist in Guava 13 (it was
renamed from Stopwatch.elapsedTime()).