Make stashNotifier a Notifier

Hello :)

Thank you for the plugin. I have built and installed it and it did its job very well!

However, there was a small problem: the **Notify Stash Instance** action appeared in build steps menu, not post-build actions menu. So it was the second build step. Jenkins does not run the second step when the first step fails. So when our tests failed, we did not get a notification in Stash.

Extending `Notifier` instead of `Builder` made the action appear in the post-build steps menu and the problem was solved.

Please excuse me if I did something wrong. I am not a Java programmer. I just used Sherlock Holmes skills to make these changes :) Perhaps something else should be done too.

Thank you.