Add a preference for the test view taking focus when tests complete

I use TestNG a lot with personal projects and in my job, and it is wonderful. One of the few things that makes it more difficult to work with in Eclipse is that the test result window jumps to the top of its pane once a test run completes, overlaying whatever I had been working on while waiting for the tests to complete (or the console I was watching while the tests ran). The standard console has a preference to disable this behavior, and it would make the plug-in integrate much better with how I use Eclipse if it also presented such an option.

I've put together a pull request which makes this behavior a configurable preference (which defaults to the current behavior). It allows the default "focus on test complete" to be turned off, similar to how the built-in console can have its "show on stdout" and "show on stderr" behaviors configured.

I didn't see any specific guidelines for pull requests in the readme, so I just tried to match the existing coding conventions. I manually tested the change - I'm not sure how to unit test Eclipse plug-in UI elements

![testngpref](https://cloud.githubusercontent.com/assets/1018131/5195720/3b46c4e2-74ef-11e4-9a36-c2434cd0560b.png)

Thanks for the great work on TestNG overall, its been a very valuable tool!