Replace deprecated TestWatchman by TestWatcher

This patch fixes a NullPointerException when using FluentLenium v0.8.0 and later with [Concordion] (http://concordion.org/).

*Rationale:* The Concordion JUnit runner creates a fake `FrameworkMethod` instance to run the test scenario. This instance does not rely on a real java method of the test class therefore `FrameworkMethod.getMethod()`. Therefore a NullPointerException is thrown when FluentLenium tries to fetch the `SharedBrowser` annotation. By replacing the `TestWatchman` by `TestWatcher` it is possible to bypass the `FrameworkMethod` and directly access the test class with `Description.getTestClass()`.