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.