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

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 `through2` stream and to emit an 'end' event when it has completed which should be listened for in the task.

Using this version, you can setup a gulp task that looks like this:
```javascript
gulp.task('test', function (callback) {
    gulp.src('**/bin/**/*.Test.dll')
        .pipe(nunit({
            command: 'C:/tools/nunit/bin/nunit-console.exe'
            //options: '-al'
        }))
        .on('end', function(){
            callback();
        });
});
```
Let me know what you think.