updated to use gulp.src('') globbing to get the test assemblies.

UPDATE: with latest commit, I switched `through2` to `event-stream.through` as it handled the switch from a synchronous stream to an asynchronous stream `emit('end')` more gracefully. This also cleans up the gulp.task a bit as you no longer need the `.on('end', fn)` or callback in the task.

I have updated the gulp-nunit plugin to allow for standard gulp.src globbing (e.g. `gulp.src('**/bin/**/*.Test.dll')`) so that the files don't have to be explicitly added to the options. I also setup the nunit tests to happen in the flush phase of the `through` stream and to emit an 'end' event when it has completed.

Using this version, you can setup a gulp task that looks like this:
```javascript
gulp.task('nunit', function () {
    return gulp.src('**/bin/**/*.Test.dll')
        .pipe(nunit({
            command: 'C:/tools/nunit/bin/nunit-console.exe'
            //options: '/xml:console-test.xml'
        }));
});
```
Let me know what you think.