Changed View.add_job() to actually add job to the view, rather than replace all jobs in the view by that job.

...though it's quite possible that I'm misusing jenkinsapi in some way.

So, I'm trying to put several jobs in view with the following code:

```python
def put_jobs_into_view(view_url, jobs):
    view_obj = jenkins.get_view_by_url(view_url)
    for job in jobs:
        view_obj.add_job(job.name)
```

After running this code there is only one job in the view -- last one.  And if I try to step through the code with debugger, after each invocation of ```.add_obj()``` there is only one job in the view, one corresponding current value of ```job``` variable.

And after applying this commit it works as expected.